The latest projected minimum deposit is 32 ETH. Even a (relatively) small fish can stake.
I'm not going to comment on a potential ROI, since the formula is not nailed down. The question is whether the interest on one's stake is greater than other options, also considering the inconvenience and risk of staking.
There is currently no cap on the ETH supply. Were there to never be a fork, there would be 5 ETH created per block, forever. Ethereum may or may not have a finite supply in the future, but if it never does, ETH will just continue to pop into existence.
The latest (technical) information on Casper can be found in the Mauve Paper