Well I've seen a 1 bed casita with living room on the ground floor & bedroom above on at ?30,000. It's immediately usable, although one would want to have the kitchen and bathroom done fairly quickly. It's in the zona colonial close to Independence Square.
I've only seen 3, so I don't want to leap in feet first, but your comments are interesting. I am serious about doing this, and the strength of the ? against the $ seems to make it a good time to do it.
I have the cash in a UK account paying about 5%, so if the capital appreciation is above this, I have an insurance policy against not being able to get over or not being able to rent it.