Both of my daughters have had birthday partys at BABW. I may be in the minority here, but both me and my DH find it much easier than an actual party at the house. And by the time we factor in the costs of balloons, cakes, snacks, and everything else we would get for a party, we feel it really isn't that unreasonable. What we did was just invite their closest friends/relatives. I think there may be a 7 child minimum. And we told each child we would be purchasing items up to $25 (tax not included, it is much easier to keep track of that way!). We chose to do $25 because that way it was up to the child to decide what to buy. They could pick a less expensive animal and get an outfit for it or they could spend the whole $25 on one animal. I figured that way they could all get exactly what animal they wanted. Of course, it seemed most of the parents sent the kids extra money which was fine, the employees at Build A Bear are amazing at keeping track of everything.
As for discounts, we ended up filling up at least one punch card and we had one almost full before, so we filled it up as well and the employees (anyone else want to type CM's everytime you type "employees"?) took the amount earned ($20) off of our party that night.
We really had a good time with the parties. They were two years ago and my youngest (she will be 9) is thinking of doing one again for this years birthday. (The kid is hooked...I think she needs a BABW support group...she wants to spend all of her money there!)