You can get portable ones (or disposable b/c I dont think you can change most of the filters) off eBay for around $65. For a nicer permanent unit that you can replace all the filters run about $120 and up. I spent about $180 on mine because I added a few things like a 3gal tank under my sink, a faucet, and a dual inline TDS
meter. I paid about 135, for the kit and the add ins were about $50. I bought mine from eBay.
Here's what I bought. I am pleased with it, it was easier than I thought to install under my sink.
I also added a Deionizer, that was +$15
meter +$25 (lets you know what your filter is doing and when your filters need changed)