If, like me, you like to take an analytical approach to what hardware to buy, you can use this handy little Excel sheet I made. It lists the most common graphics cards from the last few years, Number of Cuda Cores, Memory and Octane Bench result. You can sort by any column.
Now you can go looking for used graphics cards and enter the prices you find in Price and it will calculate the Price per Cuda Core, Price per Octane Bench Point and how much below average it is and what the maximum price is you should pay for a certain card to stay below the average.
Its a very handy little chart. Feel free to use it any way you like obviously.
Right now you can see that the GTX780ti is best bang for buck, since it can be had for very cheap. Lucky for us all the gamer kids are upgrading their rigs