Depending on the type of crank and such you have, you might be able to just toss a third chainring on your existing spider.

My new road bike came with a double, but the spider had bolt spots for a third ring. $19 for a nice Salsa granny, a few minutes of my LBS's time (they didn't charge for that) and now I have a triple.

The front der didn't shift sweetly with the third ring, so the guys traded a longer cage for it. Again, no charge, but I gave them some beer...