well, we've kept ours on budget by using the backyard and having a few high schoolers from our church help out with games and crafts. So I don't have any idea what the rates are for various party venues but can tell you that the some places my kids have attended parties at besides Encore are Pump It Up and Sky High Sports (both in Concord), Kids'N'Dance, the Art Room and Color Me Mine in Lafayette, All in the Kiln in Walnut Creek, Gymboree at the Lafayette Community Center, and the seasonal ice rink in Walnut Creek.
edited to add: a couple other places to consider: the Lindsay Wildlife Museum http://www.wildlife-museum.org/education/birthdays.php and the Animal Rescue Foundation http://www.arf.net/people-programs/birthday-parties/birth...
You might also want to check with the Parks and Rec departments for Walnut Creek and Concord to see if they have birthday party programs - I remember when my girls were in Kindy, one of their classmates had a b-day at the Orinda Community Center/Park that included a cheerleading class in the community center.