I'm a little concerned that you want to try and do this "do this as cheap as possible". Aviation is not a cheap hobby and never will be. During your PPL training is the worst time to try and cut corners. If you can't afford to do it properly, wait until you can...
Have you tried asking around your flying school to see if someone can lend you some books? You won't need them all at once, so maybe you could share a set with another student? You might also find studying together helpful.
Oh, and don't bother with the Thom book on R/T, get hold of a copy of CAP413 instead.