You use the word as to indicate that you are comparing a property of two things- either to say that they are the same , or to say that one is less . If you mention two things, you use two as:
My car is as fast as his [car] - equal
Ice cream is not as nice as chocolate - less
If you have already referred to the second item, or the listener will understand which one you are talking about, you can omit the second as and the second item:
His car is very fast, but my car is as fast - equal
I love chocolate, but ice cream is not as nice - less
This is the structure used in this sentence: the second item is the store your are comparing to: maybe the store that you are in as you speak.
The guitars in the other store are of a higher quality but not as cheap.