I'd like to add a few additional costs here you overlooked and explain and give more in-depth context to a couple of others you mentioned as to what all this entails:
1. FCC approval
Absolutely necessary to be available on carriers, and unless you want to be fined out of existence and sued by the U.S. Government for selling an unlicensed device you'll do it. Without this, your devices are effectively dead before they have the chance to be released, and requires some serious industrial engineering to ensure your device meets all pertinent U.S. laws against radio/RF interference, radiation, and general safety.
2. Retail distribution agreements
You can't go anywhere without devices available to be touched and experienced prior to purchase. A mail-order webOS phone will die on the vine, and after the Touchpad you'll be kissing feet left and right to even get yourself in the door when you utter 'webOS' to an executive. And, what's more, your best vector to immediate U.S. brick-and-mortar retail penetration apart from the carriers is the same outlet that felt screwed by HP the hardest: Best Buy.
3. Carrier agreements
You did touch on this, but I'd like to expound in more detail what this involves... This not only includes retail presence in cellphone retail outlets, but also support agreements (including updates), carrier-specific branding (including but not limited to specialized applications per carrier), promotional agreements (including commercials which, as you know, don't come cheap), and subsidizing. And a LOT of specialized legal involved at every step. This is an
extremely expensive step for a new device manufacturer to undertake, and don't count on Sprint offering you any favors--they've despised webOS since the failure of the original Pre to truly become "their iPhone." Outside of the original Pre and (crippled) Pixi, they haven't looked back.
4. Marketing
I work for a revered digital marketing agency, and you're looking at a couple million dollars here for a proper product launch and continued hard support. And that's just on the digital side. Not joking. I haven't even touched on the print/television side of things, all of which you'd need to get the mind share to sell a couple of million units. Look at how badly Samsung's doing in the tablet space, and they've sunk hundreds of millions into their Android tablet marketing and manufacture already, if not billions...for a sliver of the tablet market.
5. Sourcing components
Rule #1: You do not have the supply chain prowess of Tim Cook; nobody currently does, which means you're at an immediate disadvantage in pricing strategy. Accept this. Be prepared to buy components in huge bulk at a somewhat elevated price compared to your competitors, and ensure each one of those parts all work in conjunction to make the device as easy to work with webOS in its Open-Source state as possible in order to lower the costs associated with device driver development as that's where your savings will be made in the short-term. And be prepared for more contracts and to station a few employees in China to do the buying and ensure quality control is being met. And that means more money spent on lawyers, like most everything else mentioned here.
6. Manufacturing agreements
You'd better be prepared to set a number on your initial allotment of devices, because factories don't pump out small quantities at low prices; the value comes in larger quantities that involve mid-term contracts to provide the initial amount with the obligation that you'll stick with them for subsequent rounds of manufacture. Unless you're looking for cheap Shenzen products, this is how it goes. You're buying bodies to build everything, and the machinery and space to make it all happen. Extremely expensive, and again, expect a few employees to be stationed nearby the factories at all times. Can't forget the lawyers, either.
Again, accept the fact you aren't Apple, Samsung, or even smaller China-based manufacturers like Huawei that can get lower-cost bodies due to high-volume or proximity. You will pay considerably more for this as a n00b to the industry, and that's just how it works.
7. Ecosystem
You'd better be more than prepared to provide a digital ecosystem the likes of which Google, Apple, and Amazon already provide. This means first-party music services, eBooks, movies, cloud storage, productivity, compelling applications and more all tied together to work in conjunction--or at the very least as seamlessly as they can be. No contracting out all your stuff to a disjointed set of third parties like HP did. Be prepared to negotiate contracts in all areas of consumable media to make this happen, and by god, it ain't easy nor cheap in the slightest.
You also need to get the big players on board prior to a relaunch in order to attract people to the rejuvenated platform, and that usually involves paying them to build an app or paying them to allow you to do it yourself. And Netflix doesn't take requests, as you've seen. And people see Netflix and the first-party services mentioned above as cornerstones to a successful platform whether you like it or not.
Regardless, this takes time and a hell of a lot of developers and money. When you look at the mobile landscape today, what you see in the lead are those platforms that have a cohesive ecosystem already developed and out there.
And you need best-of-breed to survive and flourish.
8. Patent licensing
Man, you are so screwed if you don't do this. You will pay patent license fees, simply because HP doesn't own every patent required to manufacture a webOS device license-free; if someone claims you won't, they're fracking naive. And this doesn't even include the patent trolls out there that'll try to eat you (or the application developers) alive unless you have legal specializing in IP and Patents on this like a hawk for each region you retail your device. If webOS ever becomes big enough to pose even the slightest threat, add a pretty large force multiplier to the likelihood of being sued over patents.
If ever you should doubt the importance of solid patent licensure, remember the patent lawsuits against Samsung's phones and tablets by Apple [I]even though Samsung is also the single-largest component producer for Apple's own products. Yeah, it's just that cut-throat.
I haven't even touched on the number of employees, legal teams, costs of endless prototype iterations, or other aspects of a corporation that would be involved here. If I did, this post would more than triple its size, and I've gotta sleep sometime tonight. This is all just off the top of my head here.
That said, best of luck with your effort, but I really hope you've thought all of this through. I'm not being trying to be a **** by pointing all of this out, because this is a very realistic idea of what all needs to be done just to ensure you can even get the product to market, much less ensure a continued success trajectory. Where companies like Huawei, Samsung, and HTC (among others) can ride everything out by riding the coattails of Google's massive ecosystem and patent protection in many areas, HP doesn't have any such ecosystem right now other than the patents Palm gave them along with any licensure of older Palm patents from ACCESS. It's a good first line of patent defense, but by no means bulletproof--and you aren't even HP.
tl;dr: webOS needs existing device manufacturer support to get back on its feet far more than it needs new blood trying to do so, because it's a minefield that'll cost hundreds of millions just to start.