Costs are also dependent upon where you will be traveling, how much experience you have sailing, your preferences - marina living or anchoring out.
The Caribbean is very expensive for food, and will cost you approximately 25% more than food in the US. *The exception would be Trinidad, and fresh fruits many places from the local markets. *You can find reasonabily priced repair parts and boat gear in islands where there is a Budget Marine. *Anchoring out is still the norm, though I hear that in the islands of St. Martin/Sint Maarten and St. Barths they now charge for anchoring out.
South American is, in general, quite inexpensive for food, but quite difficult to obtain imported boat parts.
The South Pacific is expensive for everything, with a few exceptions.
Australia is quite expensive for any imported items; food is usually reasonable and there are excellent items for privisioning, including the best dried vegetables we have found. *(I understand that New Zealand also has excellent dried veggies).
In general, living aboard a boat and actively cruising is quite a bit cheaper than living on land if you prepare most of your own meals, anchor out most of the time, and don't spend an outrageous amount of money on telephone communications to your friends and family back home. *Skype and a HAM license can help immensely in this area.
It's a wonderful lifestyle, though if you haven't done much sailing or cruising it takes some adjusting.