Not only the $ for the playset, but the $ to have a HUGE extra room as well. My inner environmentalist cringes
To me, it actually looks for a pretty limited age group (the slide, for example, seems like it would have to be pretty small). When my kids were younger we had just some of those pop up tunnels, an inflatable bounce house and one of the foldable slides. Oh, and fort building supplies.
Do you have an IKEA? They have really inexpensive indoor gym equipment (like rings & stuff). You could also make your own "rock wall" right against the wall--- just make sure you are attaching to studs! I bet you could also build something to drape play silks over and get the feel of the fort, etc.. I am not skilled at all, but it would be a really interesting idea to contact say, a local high school with a wood shop, and see if there would be some people interested in a BIG project!