Return of Investment - Rendite, die
That's the easy part.
And then there's the math part.
Between a rock and a hard place - zwischen Baum und Borke (außer für den Borkenkäfer). Borke - the bark of the tree . And Borkenkäfer - bark beetle, the insect which dwells and breeds in and dines on what's between bark and stem of a tree. For many other species that would be downright uncomfortable. It would be like being between a rock and a hard place, an expression which apparently is frequently used in Australia, less so in North America.
Stichprobe - Spot Check.
Hieb und Stichfest - Foolproof.