I find one of the best ways to balance battles, in general, is to do a lot of playtesting. I also believe always having it in mind, if you always remember balancing you will think about it while eventing, adding items, making shops etc. Balancing is one of those things you always have to work on, you have to think, hey if this item is worth $5000 and the first town the player won't have nearly enough then you should choose another item or change the price depends on what the item is. This way the player won't be intimidated by the prices and if they happen to find this item, they will be rich and pay for all the armor and weapons that should not be purchased until later in the game, making them over powered. You basically need to always think about balancing so that while you're developing your thinking about the player and their progression. Balancing is a tough thing to do but if you're always thinking about it then you're always doing something to it, re-adjusting whatever needs to be, then when you playtest you can either confirm what you've done is good or re-adjust again, rinse & repeat lol.