How much are schoolmaster horses worth?

Depending on sex, age, rates?

personally dont even bother selling my school masters I just free range them for the 1k ebs

Schoolmasters (in all disciplines) can't show, do they are pretty much worthless after that, especially if retired which they usually are.

Schoolmasters train with your riders. You can still enter them into contests and have them breed with other horses. I don't know how much it's worth. It can be really expensive, but https://horseeden.com/horse.php?id=37935992, cost about 500.

Here's a school master that costs 500 and the age is 13.