I have 3 kids with computers, and all of their friends have computers, and low budgets, so I'm always trying to find a good deal on video cards. There are a lot of factors (budget, what PSU they have, case ventilation, what games they need to play, etc..) but I have a system for getting them a card for their needs. Most of the time, they just need something that will play Minecraft decently, and the minimum I've found for that is something with the performance level of a 9400GT. If they play fps games, then they need something a little better.
I use the passmark website to get a general idea of the performance for various vidoe cards. It's just one benchmark, but it's the best thing I've found to look up ANY cards performance:
http://www.videocardbenchmark.net/high_end_gpus.html
I keep an excel spreadsheet with values from common cards, especially cards that are usually a good price on Ebay. Here is a basic list of crads, and their scores:
AMD cards:
R9 270X 4518
HD 7770 2151
HD 5830 2060
HD 5770 1684
R7 250 1412
HD 4770 1067
HD 4850 1034
HD 4830 965
R7 240 960
HD 6450 288
HD 5450 215
HD 6350 216
NVidia cards:
560 Ti 3545
650 Ti 2682
GTX 460 2654
550 Ti 1922
GTS 450 1550
GT 730 926
GT 720 768
GT 430 649
9400 GT 197
Then I look up the cards on Ebay and see what I can get. For a minecraft system, you need at least a score of 200, so the shellshocker GT 720 deal for $30 would be an awesome deal. Problem is, you can't always get a good deal when you need it.
I typically can find GTX 460 cards for around $45, which is really good bang for buck. At the really low price range, you can often get the HD 4850 for $15-$20 dollars (the last one I bought was $12 shipped). For $100, the R9 270 is really good.