events with 20 to 30 buck entry sounds about standard to me, my experience being assorted stores in the northeast having monthly events on the weekends in that range that usually got somewhere between 12 and 30 players. payout was usually to top 4 or 8 depending on event size, with the bottom half of that getting roughly their entry back, or more for the larger events.
I think 1ks at one store were 400/200/100/100/50/50/50/50, but that's at over 30 people with $30 entry. comparably, a 16 man event for 25 bucks paying out to top 4 could be 200/100/50/50 or so; if you hit 24 guys you probably want to payout at least entry fees back to top 8, something like 240/120/60/60/30/30/30/30 for a $25 entry.
payout was credit at almost all stores, which is fine as long as a store has a decent singles case for vintage/legacy purposes and a bit disappointing if not, but I'm sure people will be happy to play for cash. I'm not sure what situation you're coming from with regard to the potential venue, but I think hosting events that pay out in store credit makes it a lot easier for stores to justify scheduling, space, and promotion for events than events that pay out in cash; I've definitely seen stores that have prize pools in credit sometimes larger than the cash received for entry, which is obviously easier to justify when not paying out in cash since you're basically giving a discount in exchange for a decent amount being guaranteed to be spent in the store.