I freely admit that I don’t really “value” my silver ranked mares much unless they’re young/relatively unproven, have really awesome/rare bloodlines, or sentimental value.
That said, I don’t think a silver ranked mare should lease for more than 15-20k, unless she was super awesome on the track, is young/unproven (i.e. 1 MSW foal and a winning 3yo/unraced 2yo or something), or has really awesome/rare bloodlines.
For mares with breeding rankings, I’d probably start with 1/2 of the stud fee for that range as a baseline. So the suggested stud fee for silver ranked studs is 30-50k, which means I’d look at 15-25k as a baseline for pricing silver ranked mares. (Add or subtract to that as necessary based on age/bloodlines/produce record/race record) Unranked/bronze/newly retired studs the fee range is 0-30k, so I’d consider 5-15k a decent range for those types of mares (again, adjusted for race record/bloodlines/etc). Same logic applies for gold/platinum ranked mares.
Selling broodmares is an entirely different prospect. But, as Andrea said, foals sell pretty cheap, and you have to factor in the additional costs to get the foal raised and to the racetrack before they can earn you back any money. (Plus shipping costs to breed the mare, etc.) In reality, it’s way cheaper to just buy a yearling in December and race it 5 months later. IMO, leasing is mainly to improve your breeding stock, so the main reason you’d want to lease Mare A is because of her performance and bloodlines.
Your mare’s got an “enh” race record and decent bloodlines, hence the logic of “take 5-15k and maybe add 5k for the bloodlines”, so if she were mine, I’d lease her somewhere in the range of 10-20k, probably more on the 10k end since she stunk as a 2yo and didn’t burn up the track as a 3yo much, either. (As Andrea noted, race record is generally more telling than bloodlines, since no matter how nice the family is, if she can’t run worth crap, her babies probably won’t either, since they’d be more closely related to her than her fabulous connections.)