The cheapest I recommend is the 7750/R7 250x
The 7750 doesn't need much power but the R7 250x requires a 6 pin connector and a semi decent psu. (400w ish. Depends on brand more than on wattage)
On Nvidias side I would recommend a gtx 750 or a 750ti depending on how close the prices are.
If you want to know how these cards perform in specific games and how much they cost you can simply google it.