Your going have a tuff time finding a "inexpensive" 12V BluRay player.
They DO make them, but they are not "cheap" or inexpensive..
HERE is one for $250.. (I can't say how it works, don't own one)..
If you don't like that price then perhaps try portable BluRay players?
HERE is a portable one for $139
If that is too expensive then simply by a cheapo all in one home style player/surround system and power it from a small inverter..