I think Wii U will get more expensive with time; like most systems that weren't successful, as the amount of units produced is lower and it will become closer to a collector item, and everybody wants to make money out of those situations, sometimes going way too far, like "Flinstones - Surprise at Dinosaur Peak" for the NES, that seems to have a price of over 1700$... Or even
more!!
But, to me, the real problem going forward is that the console relays on the
Gamepad, which is a unique piece of hardware that made the Wii U, well, the Wii U. Unlike the Wiimote, it never became a standalone controller that you could buy on its own (at least outside of Japan), and officially, a replacement from Nintendo was stated to cost around 140$; so for the most part, there's a limited number of them, and is somewhat attached to the number consoles produced, making things harder as time passes and they break (either for irresponsible use or lots of use).
Ebay and Amazon show that you can buy Gamepads at minimum around 70/80$, and often going over 100$ with shipping, but those seem to be second hand controllers (and reviews tell that all don't work perfectly, like not being able to sync with a console), which means finding systems without Gamepads will be more common as time goes on. In contrast, the Wiimote still was officially being produced during Wii U's life, so there are way more out there. There are also quite a few third party Wiimotes, which, even if I personally can't recommend, at least they let you do something with the system. Third party Gamepads seem to be out of the table for now, because of how it connects with the console.
I found one at 90 euros here (alongside those 20 euros Wiis I mentioned the other day). It came with the Gamepad, which is pretty good all things considered. It's also kinda telling how important the Gamepad is to the overall system and its value. Looking at Amazon, though, I find lots of supposedly new Wii Us going for
over 700$ for the 32GB version (600$ for the 8GB one, too). There are refurbished systems for 300$ and up, which is the price for the new Nintendo Switch, a system that's more successful and appealing (and also getting most of the "important" Wii U games, like Codemastershock says).
I find all of that simply bonkers. Looking at this, it may not be a bad idea getting one as soon as you can; if you are interested and find a good deal, because I doubt things will stay at a decent price for long. Best places to look would probably be physical stores and sites like Craigslist or Wallapop where you deal directly with the owner of the system. Although I still maintain my opinion that the Wii is still worth it, specially with GC-related stuff.