But bonuses are typically discretionary compensation that will vary from year to year based on the performance of the firm and of the individual, no?

Or even more obviously: what if I advertise a sales job that pays only on commission?

Are bonuses a mystery or do you have actual metrics that you can use to gauge whether or not you might get a bonus? Are they not part of your compensation?

Who would take a sales job that doesn't list what sort of commission they'll be getting? And wouldn't average income from such a job actually be helpful if you are deciding to take it? Sure, it is wonderful if you get commission each time you sell a car, but if folks aren't getting paid very much per year, it probably means cars aren't selling as well as the what the "possible income of xxx.xx" is saying they are.

These are all things that can be disclosed and if they aren't, do you want to work in places that have secret formulas to give you pay?

This isn't as hard as you make it sound.

You know the bonuses you awarded last year. Being transparent means you reveal that. Foo's salary is X, last year Foo received Y bonus, which was Z% of his then salary.

A sales job that pays only commission still has compensation data: the commission percentage per salesperson, the commission paid during the previous pay period. This tells you what you are likely to make, and if your commission is the same as others.

Bonuses should be same for same role and out put metrics. With sales everyone should have rights to same base-pay+ commission structures. They could even choose from a few and be allowed to change them every quarter or year.