There's this (semi-)common qualitative formula for motivation that goes like: M = (V E) / (I D) where M is your motivation toward something, E is your confidence level of getting something from it, V is the combined value of the task and its outcome, I is your distractability, and D is the delay in arriving at the outcome (task length, etc). But -- even though this is just purely qualitative -- and this is extremely minor and stupid, but still: -- you can take the logarithm of both sides and get a linear combination of new, qualitatively-equivalent metrics: M = V + E - I - D. The derivative of this new equation is much simpler: dM = dE + dV - dI - dD. The derivative form can be used to determine which thing you should focus on; for instance if dE > dV then you should prefer increasing your confidence of success rather than increasing the value of the task and its outcome

Note the derivative in the typical form is something like: dM = (dV / V + dE / E - dI / I - dD / D) M, which is significantly more cumbersome. Though, this is all qualitative, not quantitative, so...

Also, note that some studies have estimated human value metrics for money are roughly logarithmic, so there is reason to believe the linear form should be preferred. Though, the original and logarithmic forms aren't compatible if I or D can be 0 (ie: value-less), or if I or D can be negative