Keep in mind that currently each block you get a *large* set of maps (a full 32 bits of search space!) to choose from for your "level."  Basically, 1MOTO would buy you one nonce's worth of individual maps... however big the nonce is would determine how many maps that is.  A full 32 bits might be too large, but it should be left sufficiently large so you don't end up paying for a set of maps that are all entirely unplayable, etc.  Maybe 8 bits? 256 maps?  (Plus there's something cool and retro about it being 8bit, yah?)  I dunno, that might be too few.... there are some other things to consider.

You shouldn't have to "use up" MOTO to mine (human or bot) as this deters mining in general, and remember we want to give incentive to mining!  But yes, otherwise you are right, the idea is that the player (bot or human) should stake something to the network for their attempted play.  In HUC you stake 1 coin to join a new army to the map, and when your team is dead that coin goes back into the network.

Oh, and to all of the people suggesting captchas; even if some very fancy new multiparty computation stuff was used to securely generate/verify the captchas (as has been suggested elsewhere) captchas themselves are basically ineffective these days to people willing to burn some electricity to solve them.  (They are still effective in many cases because most attackers simply wouldn't do the necessary energy spend.  Coin miners with direct incentive, on the other hand, probably would.)  It would still be machine hashed and ultimately bot dominated.... just like every blockchain probably will be eventually.