There's a club in Somerset I've been to once that has a structure that might be interesting to you, DtG. The Old Down Acoustic Club (Litton, nr Chewton Mendip) operates every Wednesday as a singers' and musicians' club. One Wednesday in the month they have a booked guest act (plus plenty of local resident performers) at Â£2 entrance, and then one Saturday in the month they have a big concert with an entrance of Â£10. The Wednesday I went they had a 4-piece band plus 8 or 9 floor performers. And a raffle.
For the Wednesday guest spots they have The Amazing Mr Smith, Foo Foo, Beck Sian, Ember and Keith Christmas booked. For the Saturday concert they've in the past had the likes of Spiers and Boden, Carthy and Waterson, Vin Garbutt, Dick Gaughan and Steve Tilston.
I don't know how they finance things or if they charge on non-guest-spot Wednesdays.
Going back to the Topic and "phasing in" price increases - the Topic has a normal price of Â£4 and Â£5, and special high prices for one-offs (generally with printed tickets as well). Then it is back to the normal price for the rest of the year. This autumn we have a few more higher-price-than-normal acts like Julie Felix, Allan Taylor and Wizz Jones to mark our 50th anniversary (club founded in 1956, you know) but other evenings it is back to normal. And singaround nights are free.
So you don't have to "phase" a price in. Just make sure that for any costly concerts your normal punters know well in advance that it is more expensive and they won't see Vin Garbutt, for instance, for Â£2.50. Your higher-priced/better known acts will draw in non-regulars too. You don't want to scare off your regualrs, so a separate day, like the Old Down does, or a bigger venue for special concerts might be a good idea.
I guess you ought to phase in booking the more expensive acts, though... a club that has booked, say, 25+ acts up to 36 weeks ahead has made certain financial commitments of several thousand pounds that it won't know for sure it can meet until the punters pay on the door!