Outdoor cat enclosure

So my cats love the outdoors but because they are exotic, I don't let them out unless they have a harness on and I am watching them. I want to build an awesome cat enclosure for them to relax and have fun. Does anyone know roughly how much it would be to build one/buy one?

I have the purrrfect cat fencing system My 6 cats love it.its good size and escape proof .Just google the purrfect cat fence this site used to have a add for it..

Here's an article about cat enclosures to help give you some thoughts/ideas about enclosures:  http://www.thecatsite.com/a/cat-enclosures