I do not think that there is one to be honest. If folks continue to buy cards and use them for Litecoin mining at the current rate then I do not think that any of them will see a positive return on the price of the card. As long as AMD GPUs continue to be snapped up as soon as they reach the shops, I expect that the card manufacturers will churn out product as fast as they can and we will just see more and more GPUs getting lower and lower returns.
Of course, it you have another need for a card (gaming) then buy one good card and mine with it for a few months to pay for a part of the cost and then you still have a card for games or whatever.