My ultimate goal is to make a robot that others want to buy. I want to do this for a living.
You could start out by making those $20 linear actuators. If they are any good, I'd be a customer.
This, too; I just ordered one of these for my UGV project:http://goo.gl/vbBle
24 volt, 6 inches of travel, though fairly powerful (4000 N - definitely need limit switches and such with that kind of rating!) - still, $34.99 (I misquoted the amount before, my apologies). I don't really consider this a lot of money, though, for a used commercial linear actuator.
Even so, I would love to see a cheaper source for hobbyist linear actuators.