Everyone has a different range of what is affordable. I know many people that think Eskadron boots are affordable and ok for every day use (Eskadron has a wide range of products and prices), Thinline and Woof also make boots like this. I also know many people that are very satisfied with a cheaper "knock off" of the Eskadron style such as Dover-Pro Boots or Roma.
Ultimately, any of these are fine options for schooling boots, it's totally personal preference. I've learned with most horse things, you get what you pay for! Another thing to consider is whether or not your horse has any lumps such as splints on his legs, because that can cause rubs with harder, molded boots like these and you might need something like Equifit T-Boots to accommodate that.