You sound like MacGyver. I hope you're not saying we could all get our standing rigging replaced for something like $40 and the fact that you did it once, you don't mean to say: you'll be able to duplicate the bargain the next time you need it replaced. Which maybe sooner than normal.
Anyway the OP is pretty vague. Obviously a gift of a boat or free use of a boat would be cheaper than anything, initially. And hey, if you can do it once you might be able to do it again.
However, using the all things being equal approach to logic tells me a low end rental would be cheaper than a sinking boat. It May not be cheaper than a gifted boat, but then that would not be all things equal, would it?
Let's see: if the stove goes out, its the same. If the refer goes out, the same, the toilet same, Paying the utilities, same. Replacing the $40 rig, the $50 diesel, the thruhulls, the nav. lights, the any other boat part or system, Not the same! An apartment doesn't have them. As for the roof, that's the landlord's.
You can take any logical shortcut you want to justify the economics of a live-aboard, but it's just self delusion. Why encourage it?