Back to more of your OP - even 100 rds a week can provide outstanding training value but plan it and work on skill development. Not just dumping rounds without focus.
For training there is no reason to buy other than the lowest cost fmj ammo from a major brand: Remington, Winchester, Federal, Speer, Magtech, I've been satisfied with all of those for training purposes.
That's based on a 6-15K rounds/year habit. Depending on circumstances.
Near where I live I can buy a 1000 round case of 9mm training ammo for under $200 right now. For smaller bite sizes, Walmart seems like a consistent good source.