$1600 for the pair seems cheap to me. Source? I say this because the Infusion Mk. 2 long boards are $1200 EACH. I think there may have been a first generation Wildcat board that was shorter? Those I can believe would be $800 a piece?

As to building your own, I've considered it but decided against it. I currently have $2K in tooling and easily another $3K in carbon lying in my shop to build 60" molded model airplane wings, so I have a pretty good handle on what manufacturing carbon things costs. There are several techniques to build good foils but the issue is we are now in a super high load case with a relatively thin foil. To build these strong enough, high modulus pre-preg carbons are likely in use, along with compression molding techniques. Even if wet layups are being done, through resin infusion or simple hand layup, and they are building a somewhat hollow structure, you still need molds capable of post-cures in excess of 175 degrees; i.e, you need aluminum molds. Lets add up the costs:

1) Set of aluminum daggerboard molds: $2K for the aluminum, plus machining time, so ~$7-10K by the time you are done.
2) Machined end-grain balsa or high compression strength foam: Since you're doing it twice, maybe $500 including machine time.
3) High Modulus uni-carbon: $200 worth might cover you
4) Regular carbon for rest of board: $150
5) Gallon of MGS epoxy: $250
6) Pint of gelcoat: $40

Still think its cheaper to build a set at home? Hotwire foam core won't work, melts in the oven. Maybe you can bag a set over a machined core but you still need several hundred dollars worth of materials, and several hundred more dollars worth of vacuum equipment.