I think the last place that's paid should at least get their entry fee back. I really don't like paying first more than twice as much as second. I think a 3:2 ratio is about right there. Between those two factors and how much you want to weight first place, that should set things. Take half an hour and work out different scenarios. And like others said, be ready with the payout info before the tournament starts. Rounding is good.
At one time I had a table of the payouts for all possible numbers of players for the $10 weekly tournaments.