I don't know if this is still valid, but my wife and I (UK citizens, sailing out of Hawaii) are two weeks from heading off on a circumnavigation. We have (after asking the same question here!) budgeted for $1000/person/month, with a sizeable "would rather not spend it if we can help it" reserve.
We are sailing in the Pacific and Indian Oceans and Red and Mediteranean seas, mixing pricey places (Polynesia, Oz, The Med) with less pricey ones (PNG, Madagascar, Tanzania). We are planning to avoid marinas and living off the land/sea as much as poss.
We have allowed ourselves two years' worth of money on a trip approximately 18 months long. We have also invested heavily in medical insurance for outside Europe and have brought a LOT of tradeable goods for barter in the more remote places.
My guess is that N America is out of the question on your budget, but that leaves an awful lot of paradise that you could afford!
See you out there chap.