Back in 2008 ASDA did quite well selling a bouncy castle which allegedly caused an uproar according to their press release (http://your.asda.com/2008/5/15/asda-brings-an-end-to-bouncy-castle-inflation). They've updated the model to include a slide and added a few quid in the process.
I've been after a "proper" bouncy castle for the kids (honest) and this is the cheapest by far. Some stores have them in stock, although you can get it delivered to your local store free of charge.
This Happy Hop bouncy castle with slide will keep your kids entertained for hours on end. The bouncy castle is quick and easy to inflate and can be ready for endless fun in just 1 minute! Features a surround safety net for your Childs safety and your peace of mind.
The bouncy castle comes flat packed and includes a polyester carrier bag for easy storage after use. Also includes a 220-240v air blower for inflating the bouncy castle, simply lay the bouncy castle on the ground and attach the air blower, securing with the straps, turn it on and watch it fill with air within just one minute!
Don't forget an outdoor extension cable.