Just to add to Warwick's comments, there is a further restriction on building within designated Natura 2000 areas, in 2011 the then Greek government passed a law requiring a minimum plot size of 10,000m2 to build one house in a Natura 2000 area. My own property is situated within a Natura 2000 area but I was able to purchase the 2,500m2 plot and obtain building planning permission before this law was enacted. You can check Natura 2000 areas in Crete on this website:-http://natura2000.eea.europa.eu/#
Regarding the 'going rate' for land, as I am sure you will discover, the going rate is what the seller believes the land is worth, the asking price is more likely to be based on sentiment or personal circumstances rather than the perceived commercial value you or the seller may place on the land. In my experience much of the uncultivated or abandoned land here has multiple ownership with one or more of the joint owners living on the mainland or in another country, trying to track down the owners is a nightmare, getting them all to agree on a price is nigh on impossible. If you walk around the old town in Rethymno you will see many derelict properties, derelict and un-saleable because one or more of the multiple owners cannot be traced. It's my understanding that the City is now trying to apply the equivalent of compulsory purchase orders on some of these properties so that they may be restored and developed.
I wish you luck in your search for a plot of land, I am sure your policy of asking around the village will soon alert any possible sellers - I was able to identify and purchase an olive grove adjacent to my property in this way. For what it's worth I finally paid less than half the original asking price.
It's perhaps worth repeating the common-sense advice to check thoroughly that the seller does indeed own all of the land they are trying to sell (although this is rarely entirely possible). A year after building my house a dispute arose with a local shepherd who claimed a 10m2 parcel of my land was actually his land. The disputed piece of land was of no consequence to him or to me but for him it was a matter principle, fortunately we were able to settle the matter in the time honoured Cretan tradition, a bottle of Single Malt changed hands, honour was restored and I have a supply of copria for my olive trees.