So there's a lot to read before asking a *completely* educated question about Ethereum, but hopefully I'm not jumping the gun too much...
Because Ethereum has a Turing complete scripting language, you cannot necessarily know how many cycles a script will take to execute, but Ethereum charges a fee based on how many cycles (past 16) that it takes your script to execute, how are you handling that?
If the contract doesn't have enough funds to fully feed it, my understanding is that it will cease execution. After all, if you're a miner, why would you do work for no fee?