Pick the string sizes you want for your set and order 12 bulk strings of each size from:
http://juststrings.com/ With nickel wound and plain strings, you should end up in the $3.00 per set range.
I just did that to make a .017, .020, .028, .038, .052, .070" set for my Johnson tricone reso for Open D and Open G. (Which string set I love the sound and feel of.) Because the .070" strings were much pricier than the others, I ended up paying about $52 for 12 sets, with shipping. If you don't go over .060" on the bass string, you save a bunch.