I foyu want to biuld a red lser, that can be cheap, but building a green laser is VERY hard, and deffinatel not cheap. check out http://sales.stonetek.org for cheap red diodes and modules! Welcome to the forum!
assuming you already own or have access to a soldering iron, a digital multimeter and other basic tools your cost on a red will be around 50-60 bucks if you are patient. if you don't have the afore mentioned items the cost will be around 125-150ish because of the tools needed.
However the good news is if you do buy the tools your second one will be cheap =)
yeah greens right now are NOT cheap to build, but i'm working on finding a way to make a DIY green.