If you are even a tiny bit crafty, you can make a dollhouse from a bookshelf or other old furniture. We're converting an old wardrobe into a two-story dollhouse that fits American Girl dolls as well as Barbies and such. It was just kicking around the basement and we are using supplies from our own house to decorate it. Cheap and easy, and much sturdier than the store-bought ones.
I'll try to remember to post pics when we're done!
I'd love to see it. I'm not sure I'm crafty enough to do something like that though.