As far as the performance jump goes, I don't think it's particularly worth it. As for cheapest, it will be decently cheap, but you could get a 1050, or AMD RX 470 ... all under 200, different performance by increments
A 1060.
Performs better than the RX 470 and is priced only a little bit above it.
if you can save up the 6GB windforce model (255) is very good, and comes with a free mouse atm, but the 3GB will get you by just fine. ($205)