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?
