Hi "sailor77" - a couple of thoughts - if you're going to have a land base, you can probably add your liability to that and avoid the extra insurance costs SD mentioned. BTW, some of the costs he mentioned like slip fees and maintenance, are general boat ownership fees, not specific to liveaboards. The rest of the answer is "as much money as you have." When you move aboard, what are you going to do about communications? Will you hardwire a land line or increase the minutes on your cellphone? Where will you get your snailmail? buy a PO box, or will the marina accept it? Will you be pumping your holding tank more often, or use the 'bag and bucket'? Will you do all your own maintenance (in which case, 5%/year for coastal cruising may be a better estimate than 10% IMHO) If you still have a land-based career, you may be using a laundromat more often. Oh, yeah, and your entertainment bill may increase as you start hanging out with a community of liveaboards ;-)
That said, it's a really fun life! We've been living aboard a 33-footer year-round in Annapolis for 4 years and no way would we go back to land.