Jared Spool wrote an article about how to prioritize using a formula he attributes to Bruce McCarthy.
(Value ÷ Effort) x Confidence = Priority
“The beauty of Bruce’s formula is we can make this as simple or detailed as we’d like. A simple way to represent Value is 1, 2, or 3, for low, medium, or high.”
“We want to prioritize work that offers large value while taking a small amount of effort, assuming we’re confident in both our estimates of value and effort.”
“By itself it’s a meaningless number. However, when we calculate other design item priorities the same way, we can see what we should work on first.”
I think the 1-3 scale makes this workable. Quantifying actual value and effort would be ridiculous, but all you really care about is relative value and effort between tasks you are prioritizing.
Articles of Interest 