What your talking about is commonly referred to in economics as diminishing marginal returns. The first $1000 is much more useful than the second. Also, your

What your talking about is commonly referred to in economics as diminishing marginal returns. The first $1000 is much more useful than the second. Also, your interpretation of "expected impact" is what we call economic utility, which is commonly put in a cobbs douglas utility function (utility = (some_number > 1)^(1 > some_number > 0).