I have a mate in the diamond trade in the UK and he reckons a really good quality 0.5ct cut unmounted stone at the minute will cost about GB£600-700 trade price (the weak dollar makes this a good time to buy diamonds). A ring setting normally costs between €200-400 depending on metal used and the type of setting, so in total that would be about €1200ish. If the quality of the diamond is good then it's not a bad deal when you factor in dealer margin, etc. However the price of diamonds starts to drop dramatically as the quality decreases so this is really the determining fator.