BoardWalk Bakery makes the cakes for Pop Century (but you have to place the order with Pop Century) so you should have no problem getting a good-looking mini cake. BWB made us a pretty *and* tasty mini cake for our anniversary one year.
Prices for two-tiered "wedding" style cake start around $200 and go up, depending on the complexity of the design. If your group ends up closer to the 30 mark than 40, I think you could get away with a 10-inch cake. Those serving sizes I put in the first post are Disney's estimates, but in reality, you can get a LOT more slices out of the cakes than they say. For example, a 2-tier cake like you're interested in would be the same size as the cake that comes with Disney's Escape wedding package. At those weddings, which have as many as 20 guests, they only cut into the bottom tier (the 10-inch), and there is always at least half of it leftover, in addition to the untouched "anniversary" tier on top.