How much less to offer on asking price '%' ...

@MFraser, I only paid tax on the price I paid for my apartment.

Duncan:   I only paid tax on the price I paid for my apartment.

That is the experience of most buyers,  but I guess you know Hacienda have up to five years to make a claim if they believe the sale price was under declared

Hi johnzx, yes I had read about that. There is a good article with respect to that here https://www.justlanded.com/english/Spain/Spain-Guide/Property/Fees-Taxes.

I did some research on current property values and found this http://www.fotocasa.es/en/real-estate-index-evolution-of-housing-prices/. It says the average price psqm in Murcia is 1,194€. I used the wayback machine to find the average price in January when I bought and it was 1,156€ psqm. When I multiply that by my property size it pretty much matches the price I paid. Hopefully (fingers crossed) the price I paid reflects the current "valor catastral" and there won't be any more to pay. I would think that even if I did get a bargain, it wouldn't attract the penalties for 20% or 50% under-evalutions. 

I'll just have to wait and see I suppose, but I think the law is there to stop people under declaring the price they paid in order to avoid tax rather than linking it back to some inflated valuation from a decade ago at the peak of the property bubble.

@ossie1 Just looked at average property prices in Benhavis on the fotocas site and it says 2109€ per square metre