We built our own and I think it's awesome! It's about 2.5 years old now and parts of it didn't survive the kittens getting heavier or adding a third cat, but the main parts are still there (and the kitties love it!).
we used some left over plywood we had and bought landscaping timber (something like 1.99 for 8 feet at the home despot). The most expensive part was the carpet to cover the base and the top "sitting areas". But I know that we should be able to find carpet scraps for cheaper somewhere.
I did like that we could customize the color and give them a huge long pole of sisal rope to scratch on.
It's def. not as sturdy as the ones at the pet store, but we did buy one from costco, and while the bottom part of that one is super strong, they ripped off the top towers that screw in and we can't do anything to get them to stay. (it was really designed to withstand the force of young kittens, not crazy full grown cats!)
I would def. say, if you build it, remember that big grown up cats will be tearing up and down it. We didn't really think about that and that's why we lost some parts of it.
If you like DIY stuff and can find the materials cheap enough, I say go for it!