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

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.