bang for the buck
Definition from Wiktionary, the free dictionary
- (idiomatic) Efficiency; cost-effectiveness; value.
- Do you think he would get as much bang for the buck out of a fancier, more expensive car?
Usage notes 
[Vulgar] --- World War II GI slang for "liberty activities" of a sexual nature. Seeking the best bang for the buck, i.e., maximizing sexual gratification for the least amount of money, thereby saving his money for other purchases (drink/food)