In the year 2005 Spanish property was considered to be 40% overpriced. Most investors with the benefit of such advice recognised that fact and sold up whilst the going was good. It was not difficult to spot all the red flags.
I would therefore suggest your property today is worth no more than 150K. However given that it appears the market is falling more than the 40% estimate it could be more.
To take that sort of loss I would imagine is unsustainable given your reason for wanting to sell.
My advice therefore as a professional investor is hold on to your asset until the market returns. Rent it long term if you need to and think at least 5 years before the market stabilises.