How do you make a homemade dildo sex machine?

Hey,
my girlfriend and I have always been inquisitive about dildo sex machines that can be seen on many sites. I have looked all over the internet for pre fabricated ones but they all seem to be really expensive. Does anybody know where to get plans from the will do the same job or where i can find a reasonably priced one can be found. If you do know how to make one what equipment is required for it and how much would it cost to make.