We bought my wife an eternity ring when we were in marmaris in 2006, we'd been married for 10 years which felt like an eternity.

I think we paid £500 & were told it would be valued at double that in the UK.
A couple of months ago, one of the diamonds came loose (and was lost!!!!), so we had it repaired in the UK, where it was indeed valued at around the £1000 mark.
I suppose if you buy something over there, you could always visit a couple of other local jewellers with your new item and ask for a valuation. At least this way, you'd have an idea if there was anything fishy, while still over there?