Richard built mine and it worked VERY well. Actually it was a secondary reactor.
The cost, well thats kind of a dificult question. The forst one you build will run you a little more due to tool costs. But once you get all the tools, about 50$ for the materials. PLUS, you'll need, a CO2 bottle, a regulator, and a neddle valve adgustment. Thats where a little more money comes into play.
Calcium reactors ain't cheap. But, their benefits are well worth it.