The only cheap lasers you're going to find are red, that's it. Green lasers are expensive because they're actually frequency multiplied optical lasers using a diode as the starting point. Blue lasers are even more expensive. A red laser module will probably only set you back a few dollars, A blue/violet laser or a green one will cost around 25 bucks, a true blue one will set you back 50-100 or more, each.
As misterT said this is extremely hazardous to anyone they're pointed at, lasers have to be treated with care.