The mechanism behind making them is also different.
Red lasers use a diode, optics, and some electronics.
These are fairly easy to make and assemble, so red lasers are cheap. The diode, crystal and mirror have to be very carefully aligned in order for the laser to function properly. Because it takes lots of work to make a green laser, and because there is not much demand for green lasers, their cost is much higher right now than for a red laser. Having different colors of laser light is neat because they have different properties. Also, green light bends more than red when it is put through a medium such as a prism.