Here's a little info on them...
http://www.aquariumadvice.com/profiles/6_50_en.html
As for the cost, I can't remember how much they were, but they weren't expensive. But before you get any, make sure you plan on upgrading, as they do get to 44+ inches. I'm looking at a 265G tank down the road. As for co-existing, they get along great when young, but do get very aggressive to their own kind when they get bigger. Mine seem to be doing pretty well together so far, but if one wants something and so does the other, they do duke it out a little. And being both the same size, and both have the same attitude, there is no dominant one.