Mainly because the key word there is "suggestion".
The "problem" with my placing an actual "rule" or coding limit on pricing of horses is that some horses will not follow that rule.
Babies - they have no race earnings, so the closest I could come would be to average their siblings' race earnings, if any...still, if the baby's by a very nice/not nice sire, this could be invalid
Unraced mares - average of foal earnings could work, but again, who she's in foal to complicates this, and if she has no racing foals, there's absolutely nothing to go by
Stallions - studs are generally priced far higher than other horses because you can potentially get a lot of income each year through stud fees
Even with horses who've raced, pedigree plays a huge factor. You could take 2 2yos who're 5-1-1-1-1 $100,000 and whoever's got a better pedigree is worth more, even though the "rule" would limit people to selling them for a max of $50,000...if one of them is a filly from a nice line, she's probably worth more like $250,000.