I'd say $175 cuz that's what Gamestop is offering for it!
Haha, joking aside though, you could probably fetch over $750 easily I'd imagine. Since I don't believe one has ever been sold on eBay before, the only "market" you could look at would be other custom prize consoles, but prices may vary WIDELY depending on the franchise that its for.
My suggestion would be (if you're looking to sell it) list it at a very high (in your opinion) "buy it now or best offer" price and see if you get any offers? Unfortunately (or fortunately for you) there aren't a whole lot of other Xbox One consoles that are Halo themed, so it's a real collectible to those who may want one. (I don't collect consoles, so not speaking with any bias, though I do love Halo!).