Generally speaking, the cheapest for is the one you already own.
Depending on how exact your summation of it's condition is, it might be a simple fix, or you may have contamination which has caused it to seize up. Assuming the cartridge can be saved, the rest of the forks guts being replaced shouldn't run more then the cost of a used one, of unknown provenance, most likely in need of repair. So, under $150 easily.
New fork, stem, headset, etc you'd need to go non Cannondale suspension would easily exceed that.
80 to 100 mm forks (as in Headshock or Lefty) are just right for that frame....:thumbsup: