I agree, John, nobody knows what price a house will fetch until you put it on the market. All we can do is make estimates based on what other, similar houses fetched in the recent past. You are also correct that a sudden forced exodus of ex-pats would cause a glut in supply with no corresponding glut in demand so the price would be likely to be depressed further than current conditions.
The market appears to be bottoming out at the moment but even that is not a certainty. Greek banks have a massive non-performing loan book, somewhere around 35 % of all loans, much of which is in the form of mortgages. In 2015 the Greek government, under pressure from the creditors passed a new law, Law 4354/2015, changing the rules governing the sale of non-performing loans to third parties that could lead to a massive increase in repossessions and a consequent flood of properties onto the market. At some it is possible that this could cause another downturn in prices. So far investors have shown little interest in purchasing these non-performing loans so the banks are investigating alternative methods of resolving them in-house. Nothing is certain. "Prediction is very difficult, especially if it's about the future." --Nils Bohr.