Over here u can get them for £5 a pair, thats roughly $10 for 2. They are not overly aggressive compared to some, what are you planning on keeping them with?
They are a little more expensive than that by me. About $7.99 for one, but a small discount if you get 3 or more. Depends on your LFS I guess!
If you have a mating pair, they will be much more aggressive! keep that in mind.
Mine cost $6 each. They really are not all that aggressive. I've had 2 breeding pairs and the most they ever did was chase off the others if they got really close to the babies. And even the chase was very brief.
In my experience they can be put with most any community fish, though I'd steer clear of nippy fish like serpae tetras and tiger barbs.
Just as a note, I've read that they sometimes die after a few days because a lot of the ones coming to the US are from asia and they inject them with hormones. This process results in the fish becoming more colorful and induces premature spawning. It is often recommended that aquarists obtain wild caught or spawns from breeders.