You definitely want to have the trade deadline a few weeks (maybe three or four) before the playoffs start. You could theoretically start trading right after the draft if you really want; that's no big deal.
You can either have the commissioner oversee or approve trades or have the whole league look at it and give them "veto" power if a certain number of owners thinks there is collusion of some sort. That depends on whether you want a dictatorship or democracy...
Both methods work although weird trades do sometimes cause problems between owners.
How much for a trade? Nothing really...it's just a trade. You shouldn't lose waiver priority for it or anything like that. I suppose you're talking about real money thrown into a pool though. I have no experience with that although I suppose it would be about the same as a waiver move.
Hope those help.