You don't even need to buy a premade cat enclosure.

All you need is about $200 (still MUCH cheaper than the pre-made enclosures), a Lowes store nearby (or other similar store), a husband (or boyfriend/neighbor etc) that's handy with building things, and two weekends.

That's what we did. This is built on a 6 foot x 8 foot sheet of plywood and it stands 6 foot tall. It has 3 levels to it, 3 shelves, a sisal scratching post that doubles as a ramp to the next level, and one carpet covered board leading to another shelf. It's covered in chicken wire and has an old sheet for a sun shade. *sorry the pix are older as noted by Dusty's presence and lack of several cats I have now, however this was when it was just finished and the prettiest*

Front with hinged door.



Right side.



Back side.


Left side.