It may not be in the eurozone but the BGN is pegged to the euro, so exchange rates won't alter.
Most expat Brits (and others) are selling in Bulgaria now taking advantage of the British Pound to euro rates and are thus making money or at least breaking even. Good on them!
As my father (deceased) said to me: if you only lose what you would have spent on rent or holidays in another country, then that is fine. Same as for a car - if, when you come to sell, it has cost you less than renting a car during that period, then that's ok.
Besides, all property prices across the world have come down. So, you sell your Bulgarian property at 20 to 35% less than you bought? So what, everywhere else has come down by near the same amount - so it's a basic 'swap' on price.