I think it depends on the car and what level you go to.
Modding a 1997 Ford Taurus? Not so good...
Modding a Scion Tc or something with an engine that has lot of potential, yeah definately.
The Focus is a good car to modify. Many of the stock parts aren't that great and the car has proven itself in certain race circuits. I mean just changing the suspension back to how it was made for the European market is worth it.
I say spend a couple thousand to make it perform and ride like you want it to. Or spend a ton of money and make it into a show car that can make you money or get you business, or publicity, or whatever. It's the level in the middle where you are spending a lot just for personal pleasure. That's when you should take the money and just buy a better car.