Have you read your CTCS today?

Category Theory for Computing Science is a fundamental book for any person willing to be a language designer, and it's free:
http://www.math.mcgill.ca/triples/Barr-Wells-ctcs.pdf

>>5
Usefulness and price don't always correlate.

>>6
Yes. For example, Linux is priced at $0 and has more than no utility for several purposes. However, other things which are cost-free, such as dead branches, have zero or near-zero utility for those purposes. Likewise, a car is not free, but also has zero utility for many of these purposes.

In this example, I assume that usefulness must be defined over a set of desired actions in order to be defined adequately.

QE fuckin D

For these reasons, I believe it is not always appropriate to use the saying "You get what you pay for."

>>5
SICP is also free you dumbass => http://web.mit.edu/alexmv/6.S184/sicp.pdf