All you've done is replaced the ballscrew with two sets of rack and pinion. Which still leaves your statement:
You still need to use these parts, or buy/build something cheaper.Originally Posted by Bassbuilder
To extend the machine you still need to add the rails/carriages or ground rod. Whatever you use to guide the linear motion.
The rack and pinion is just part of the drive train. Same as a ballscrew, timing belt or threaded rod.
I don't see that using rack and pinion will save you a lot of money.