I agree that it is very hard to estimate costs for someone else since it depends on lifestyle choice and how much you have to spend on boat repairs - which is related to how well sorted-out the boat is to start out.
A few comments about our experience last winter in the eastern Caribbean. Our boat was well sorted out when we started (we had lived aboard for two years i NYC) but fairly complex (genset, watermaker, solar panels, wind generator etc). Not counting boat insurance (which we paid as a lump sum ~$4200) and health insurance which we do not have (we spent no money on health expenses in the year and were prepared to pay directly if need be - prices are much, much less than US, and we have coverage for 90 days after each visit back to Canada) - we spent around $1000 to $1200 per month. We could have cut this back to perhaps $800 a month but this would have meant no goodies like meals ashore and shore tours, etc.
Since you have a certain amount of money I would suggest you not spend too much on the boat purchase/upgrade to insure that you have a adequate cruising fund. Go with a smaller, simpler boat that you can afford to get completely ready than a bigger, more complex boat in iffy condition. We noticed that Europeans seemed prepared to cruise more simply than North Americans.
One of the nice feelings we had was that we could spend more if we had to - for example, if one of us got injured or there was an engine problem or something else we could not fix ourselves - of if there was a opportunity to do something special on an island.
Use the boat renewal process to learn how to fix as many things as possible on the boat. Nothing will ruin your budget faster than hiring someone to fix something that you could have/should have fixed yourself. In some places you can get someone to fix things like refrigeration or electrical system glitches. In other places you are on your own. It is a rewarding part of cruising to get something working.
Finished the circumnavigation in early February in Grenada. Have to work on a book project for the next several months so the boat will be waiting for next year.