I have a couple GTX 750ti in my mining rig for x16R mining Ravencoin. Getting 2.3Mh/s each.
https://myrandomtechblog.com/cryptomining/mining-x16r-ravencoin-with-a-nvidia-gtx-750ti/

Haven't really looked into my power costs on the GTX 750ti cards specifically as I am not in it for profit, just playing around with them right now. I think they spec at 65 watts.  But they are really cheap used and I thought of just making a rig of 6qty  750ti's for fun, and it would be getting about the rate of a single 1070ti :-) . I see them for sale cheap used in my area and might try to get a couple more.  Good luck if you try it. Hope it works out for you. I wonder if those PC recycle places get them, would be nice to get a couple more free ones.