A best bet is the Fisher-Price Trio Batcave (pictured), model no. T3832, for $14. Choose in-store pickup to avoid the $8.25 shipping fee. That's $16 under the lowest shipped price we could find. It features a movable door, snap-in picture panels, movable stairs, zip-line, and more.
Fisher-Price Trio Batcave Set, a similar product, is available at these stores