No the Green and Blue are not as cheap as the Red one. And when it comes to more powerful they are exponentially more expensive. As for the galvos I don't think it is easy to build them yourself. No offense, I don't want to underestimate your skills but this things are very sensitive devices. Ihave a very good friend that is dealing with lasers for many years and finally he bought one RGB system with all the mechanics that is controlled by a PC software. It is amazing that can produce all the colors in a laser show. But this is way to expensive.

For such galvos we were trying many ideas like speakers with mirros mounted on the cone, small motors, electromagnets but the original ones cannot be beaten...

Good luck with your project.