First off, you will need to pre-register with BLM in order to become an approved bidder, well before the auction. You must to give them details of where any purchased horse will be kept - fence height, dimensions of pen/pasture/paddock, description of shelter. You will also agree that said horse will be trailered home in an open stock-type trailer, not a 2 horse or slant load as these are wild horses, not ones you can walk up to with a halter and lead quietly.
Pricing will be all over the board. You may get one for $700 or they may go up to many thousand.
Prior to worrying about price, color, or height, PLEASE SERIOUSLY CONSIDER whether you have the ability, time & knowledge necessary to gentle a wild animal. Mustangs can be extremely sweet, smart and rewarding to work with once gentled, but the first few weeks can be extremely dangerous, even to an experienced horseman.