The expense is in development of the first boat. Try sailing across one of the Great Lakes (Are you in US or Canada?) or any distance of 50 miles or so. You will need some long distance trials before the big event anyway. Even Burt Rutan tested his airplane by having it fly in giant circles from the Mojave Desert to Bakersfield before allowing it to embark on its around the world trip.
Once you have that prototype built and working, what do you think copies would cost? Moreover, at that point, you could probably get some well-heeled sponsors -- Timex, Gatoraide, UnderArmor, Nike, etc. come to mind as companies that sell reliability and endurance. The servo and radio manufacturers would probably chip in too.
John