It's a bit of a distorted time to do the figures on live aboards. As there are not a great deal of berths currently available at DB Marina the costs of purchasing a current one are high. Naturally this will/may change in the future, in a normal world boats depreciate at an alarming rate, I would estimate a junk is worthless at the 10 year mark so there is no way you could plan on selling for a profit, ie at a price above what you paid. And the mortgage will be a couple of percentage points higher since finance companies aren't as keen on boats as they are on bricks and mortar.
But for all the reasons listed above you do get a lot of sq ft for your money and if you enjoy boats then the upside of being happy living on one may even outweight the greater expense.