A used 750ti is more than 50, but is one of the best budget-gaming cards.
One step back a GT 740 used
Or R7 240 used would be ~50£.
But you have to keep looking for these used/refurbished deals of older cards. Most of the time they are overpriced, but sometimes you can find a great deal!
Also, dont go and buy blindly. Don't forget to check if your other components will support a new GPU. H81 motherboard is OK, will do it, but also check the power supply unit etc.
The cards i wrote about in the start, r7 240 and gt 740 will be 720p gaming cards and on 750ti you will be able to play at 1080p at less demanding games. (but all low or low/medium settings max)