That alternative is the books published or republished by Dover publications. They like to take older textbooks and purchase rights to republish them as relatively inexpensive paperback editions. A very large fraction of their books are under $20, with many under $12. A few are more expensive, but only rarely more than $30.
The level ranges from suitable for high school students to graduate level and beyond.
Here's their mathematics section: http://store.doverpublications.com/by-subject-mathematics.ht...
Don't overlook the "general" subcategory. They have some wonderful problem books there, such as Yaglom and Yaglom's "Challenging Mathematical Problems With Elementary Solutions" series.
They also do this for physics, chemistry, engineering, history, economics, computer science, biology, earth science and more.