A while back the missus and I looked at buying a cat tree for them but decided that $60 or $100 seemed crazy. This was before I'd built my awesome workbench, but I said "Hey, I could build one of those once I build that workbench!"
For about $60 (cheaper if I could have gotten a cheaper carpet remnant) and a borrowed staple gun, I made this awesome cat tree. The bottom level has a tennis ball in a box for the cats to hunt (they haven't quite figured it out yet). There's a middle level just big enough for one cat if they want to be alone but still looking out the window. The top, or sun deck as I call it, it big enough for them both to lay out and look outside all day. One post has sisal rope that goes high enough for them to stretch up to and another has a second type of carpet for a different texture. Of course, there's also a dangley feather toy hanging for them to play with.