I have seen watches with sapphire going as low as $169.00. Granted, those are Swiss Legend brand, however I have 2 Swiss Legend watches and both are made amazingly well and are very accurate. These days any major brand should have sapphire whatever their price, if Swiss Legend can, THEY SHOULD!!! (IMO)
Sometimes it's not about cost. A mineral glass or in house blended crystal is sometimes more appropriate for a rough and tumble sports / diver watch because it has a very low chance of shattering with impact.
Sapphire is great until that one time you bash the watch and the crystal explodes. The old tech hardened mineral gets a scratch or even a chunk out, but still protects the watch.
You can buy a standard hardened mineral glass crystal for less than $ 5 and a sapphire is about $ 30 from part houses.
Retro-fitting a sapphire replacement might be possible, but ask yourself if it's really practical?
Personally I prefer sapphire but there are applications for other types of materials.