Canadian artist Ron Guyatt produces beatifully stylised videogame posters and sells them via Etsy. Following requests from fans he's produced three Halo posters:
Etsy page. Including shipping to the UK they come out at about £20 each, which seems pretty reasonable to me. He has posters in a similar theme for loads of videogame franchises - Zelda, Street Fighter, Bioshock etc.
- Cowboy Out.