Materials are not that expensive: $100 for foam, $160 for carbon, maybe $120 for good epoxy, $65 for 3mm divinycell for deck, 4oz glass is cheap, $60 for deck pad, plus 2 fin boxes, strap inserts, vent and leash plug, lots of sandpaper, gloves, masks, squeegees. So maybe $600 for a good strong carbon board. Which if you make it well, will be able to sell second hand for more than the cost of materials .
But setup can be costly, you need a good pump setup, and a pump failure can result in the board being scrapped, so it's worth getting a good one. You probably need some hot wiring gear, a good orbital sander, a router, shaping stands, a shop vacuum cleaner etc.
But if the setup is not prohibitive, building the board is not that hard, and you can get an outcome like this one we glassed a couple of days ago, 5.5 kg before the filler coat: