In this case, it had nothing to do with the value of the horse; it had to do with the severity of the injury. Barbaro (for example) had a rear-leg injury, which is less serious than a foreleg injury - and he only had one leg involved, not two. Also, Eight Belles had a compound fracture, which is extremely serious; even if the limb can be reconstructed (which is often not the case in fractures of this nature), the risk of infection is severe, and when bone hardware is present, it becomes *much* more difficult to control infection.
Horses can't do crutches or wheel chairs; they can't even do slings for any length of time, nor can they lay down for too long. The have to stand up, or they die. That means that any repair has to be able to stand up under their weight. Horses bear 60% of their weight on their forelegs, and Eight Belles didn't have either foreleg to stand on.
Fractures that occur in horses can be extremely severe; there is a tendency for the bone not just to break, but to shatter. This is becaue bone is a piezoelectric substance, and under full load, the microstructure of the bone can become extremely brittle. The bone is also quite heavy, so if it becomes brittle and fractures, it's almost as if the bone explodes from the inside, creating very complex fractures. This is exacerbated by the fact that these horses are running hard, and it's very difficult to pull them up and stop them with all that adrenaline and speed and power thundering down the back stretch.
Financially, a really good stallion will generate more income than a really good mare; this is because a stallion can cover 40 mares a season, but a mare can only produce one foal a season (at least, under the rules of thoroughbred racing; artificial insemination and embryo transfer are not permitted.)Whether that means the stallion is more *valuable* than the mare is subjective, but it is true that a stallion will produce more offspring (and therefore potentialy more income) per year. But as I say, any colt with the injuries Eight Belles suffered would share her fate - the injuries themselves dictated that, not the value of the horse.
