The keeper I'm in does it a little different than anything I've seen posted in here yet. We don't use $ amounts either. It's a 20 team league with 23 roster spots. We're allowed to keep 8 players and break it down like this to promote trading. We've run up close to 50 trades already this year before the draft.
4 lifers-can keep as long as we want, or trade/drop
3 one year players-we can keep one season, and either have to promote them to a lifer spot, trade them, or lose them to the draft pool prior to the next season.
Then we also keep one rookie.
I'm also in another keeper with no restrictions on keepers. Nobody does much trading and the league is really quite boring. There's always someting that can be done to spice up a league.
Any fool can piss on the floor....may you be the hero that craps on the ceiling!