This is an interesting exercise for people who think that anything can be measured in dollars and cents. A one dimensional scale does have some trouble coping with the real world.

The author could've tried to make the one-dimensional scale work by polling people how much they'd pay to get out of a 1 year prison term, or alternatively, for what sum they'd be willing to go to prison for 1 year (most likely the two sums aren't equal). Maybe there are studies to that effect somewhere?

You still have the problem where crime X (let's say stealing a car) and crime Y (maybe shooting someone in the leg) result in the same prison term, when normal people would say that one of the crimes is a lot worse. Prison plus fine is barely multidimensional, and converting prison to money reduces it to one dimension again.

In a study like the one you propose, they should also see what effect the crime causing the prison term has on the valuations - will time in prison for rape cost more or less than time in prison for murder?

What do you propose as a better metric for decision-making? Or are you from the "if it saves only one child then it's worth it" school of thought?

In my opinion the justice system should aim to convert criminals into productive members of society, and protect the rest of society from said criminals until they're fit for society again.

Since it's not a simple problem to solve, I don't have a better solution than "use common sense and take as many aspects as possible into consideration, and dont' forget to look at the big picture". Saying that money is the only thing that we can measure and so it has to be the basis of any decision is to pretend that it's a simple problem.