The value placed on a TB mare for breeding tends to be a combination of several things.
First, pedigree. If she comes from a strong family and is closely related to winners, or to the producers of winners, she has value to a breeder. Some canny breeders will look a bit further back in the pedigree and spot a family that has gone off the boil or isn’t as fashionable and obtain what then proves to be a good broodmare for a comparatively cheap price. But it is pedigree that mainly sells youngstock at the sales so the close-up pedigree of a mare is very important to commercial breeders. Some ‘blue hen’ mares have a fantastic ability to produce exceptional runners and such mares have huge impact and their offspring command substantial prices at the sales. Look up ‘Urban Sea’ and see what she produced. Interestingly, these mares are often top class winners themselves.
Second, performance. If she has a good race record then that can/will play into her value though pedigree is generally more highly valued. Many very successful broodmares have a very light race record before they go to the paddocks. For a potential stallion, it makes sense to build up a CV based on multiple wins in prestigious races over a season or two. For the fillies, unless they become superstars like Black Caviar or Enable, it isn’t worth racing them too long. They can have only one foal at a time rather than the hundreds achieved annually by the stallion so fillies retire to the paddocks young. On the other hand, some well bred fillies prove to be just so slow even the most optimistic breeder wouldn’t bother. I know of one blue-blooded mare that can’t keep up with a 10hh Welsh pony.
The third element is the “physical” because a horse with a poor conformation will not stand up to the rigours of race training. A potential broodmare should be a good “physical” so as to minimize the chance of producing a foal with conformational faults. The physical appearance of the youngster is the other key part of success at the sales.
There are also times when a mare with good potential for breeding slips through the cracks because their owner and/or trainer have no interest in breeding and just want rid of her. I recall a story of a mare being sold on for good money after she was traced to and purchased cheaply from a riding school because a younger full brother proved to be an exceptional racehorse.
Probably the majority of TB females go on to the paddocks but only a tiny fraction of males go on to be breeding stallions.