Hulls are not the expensive part of a homebuildt boat. All those small items, like blocks, tracks, ratchets, cleats really add up when you are fitting out your platform. The rig also costs the standard pound of meat and blod if your goal is a cheap boat (dont think we should convert that phrase to "SI" metric, Wouter ). The more you can build yourself, the more money you can save, but then "time == money" and you want to get sailing as well..

If you have the time and inclination, you can build everything (except for running rigging) yourself. Foils, ruddersystem, mast, crossbeams etc. But be prepared to spend quite some time in the building shed.

If you are serious about a boatbuilding project. I suggest you start with finding or sourcing the rig/mast. That's usually the most expensive part, and the one you have most trouble shipping. It might also be smart to calculate how much you will spend on fitting your boat out, to se if you can afford it.
Looking around in papers, E-bay, Catsailor-magazine etc. for badly damaged boats you can buy cheap and use as donor boats might also save you quite a bit of money. Even if the hulls are shot, you can still use all the gear+mast and perhaps ruddersystem, foils etc.

If you start building, dont put a value on your own time. You will soon realize that by working a second job instead of boatbuilding you could have bought a new boat in a shorter time.
In sum, build the boat becouse you want to build it, not to save huge sums of money.