The reason the prices are different is because it was common practice to declare the council/belediye's average valuation on the tapu - as with John F. The way for the belediye to judge the average property sales price was totally inefficient (based on the tapu! bit circular, that!) and also only updated once every few years. You pay your buying tax based on a percentage of the DECLARED value on the tapu even if the actual sales price is different.
Now in areas where true purchase prices have risen a lot, like Fethiye, there is a huge discrepancy between the actual amounts of money changing hands and the amount of tax coming in.
What I read in Today's Zaman is that the authorities are cracking down on this, particularly in Fethiye. But the article is no longer online so whether it's inaccurate or they pulled it for another reason I don't know!
What I do say, as an estate agent myself, is that while we have received no notice that you shouldn't declare the belediye value on your tapu, the Office of the First Economic Counsellor's website (ie Turkish embassy in London) say pretty darn clearly that you should ONLY declare the actual value - so my advice is ONLY DECLARE THE ACTUAL VALUE!
http://www.turkisheconomy.org.uk/bu..._taxevasion.htm
They also imply they are trying to find out if you have previously underdeclared the value of your property and that you should come clean now if you have.
I have worked in the estate agency business here for several years and have witnessed competent, professional and honest lawyers (yes, they exist!) advising customers to declare the belediye value on their tapu and I find it hard to believe that they were deliberately misleading their customers! Also, if it was previously illegal to declare the belediye valuation on your tapu, why was this not part of the estate agency licensing course required to operate as an agent here in Didim?!
If there are any property lawyers online, perhaps you can give your opinion!
So on retrospectively declaring the true price of your property I have no recommendation. If the authorities have only belatedly realised this loophole it seems a bit unfair to retrospectively punish people who used it in good faith.