cheapest 1150 cpu that won't bottleneck 750 Ti

Solution
A decent rule of thumb is to budget 2x the cpu cost for the graphics card.

Using that rule, I would look to the upcoming Pentium K which should sell for $80
That depends on the games, output resolution, detail settings and your personal definition of how good is good enough.

Games with low CPU requirements like Diablo 3 would play fine even on a Pentium or even a Celeron while games with heavy CPU requirements like multi-player BF4 can show some performance scaling all the way to i7-4960X.