This is quite a common question that falls into the how long is a piece of string category as a lot depends on what standard of living you have on the boat plus a myriad of other boat and life related influences.
So to find a pair of liveaboard who had recorded their complete (and I mean complete) expenditure over a year is a little goldmine of information for anyone planning on living afloat full time.
So click the following link for the best free, unbiased and most importantly accurate account of the cost of a years living on a boat. James and Debbie on NB Lois Jane
When we have discussed such an option I estimated £12,000 so I was pleased that my guess was close to the detailed reality from James and Debbie. Of course there are other things to consider, such as a retained house with all its associated stand still costs even without a mortgage. Then if you wish to retain a mooring to come back to periodically for us that would be a further £1500
For me it does prove the argument it is cheaper to live aboard, but then again it depends what you live in on the land.
Thanks Debbie and James for being so detailed and open in your life afloat, as I said in my comment one of the best most useful and interesting blog posts I have read for a long time.