Depends how good you are
I didn't lose many to be honest but they don't last long as the ones you use, if you want to get a good spin of the club face have a relatively soft skin. I would change the ball after the round. However the old ones can be used for range practice.
I used to plat 3 times a week but would use same ball when not in a comp. Comps usually saturday and sunday.
You can get them a lot cheaper though and ones that are more hard wearing.
Target shooting is the cheapest sport i have ever done to be honest. Other than sports that don't require much kit such as tennis and squash etc.
Used to do downhill mtb
protection alone £500
+ travel + entry fees!!!
I have a very understanding missus