If it's a King-sized box spring that you need, they actually take (2) twin sized box springs. There is no "king" sized box springs per-se. I've found box springs @ BJs wholesale club for $59 (twin size). Full and queen sizes were a bit more.
To save more money, you could just look for a "foundation" for the mattress. Foundations are made to look like box springs, but are just a "box" designed to support the mattress.
Mattress places usually charge $29+ for frames. A twin/full-sized frame starts at this price and the other sizes go up in price from there.