Wouter has it right in my experience. The hulls are not that expensive, especially not if you build them yourself in ply/epoxy. But when you begin to buy all the small fittings and blocks, you will be astonished at the bottom line (I was).

Best option if on a tight budget and you want to homebuild, is to find an old beachcat, preferably with one or both hulls wrecked and buy it for the fittings, rig and beams. I have all the equipment from our first Tornado (hulls are history) in storage, and I am not selling..
Carbon cloth can be bought at a discount at manufacturers. They often have leftowers from the looms, if you can get in a position to buy it. Carbon tubes, "I" beams and other profiles can be bought from 'pulltrusion' firms who manufacture them for the industry. The challenge again is to find them and convince them to sell you the parts you need (if they make them).