If you don't have a pro shop, where are you planning to buy the ball, and who is going to drill it?
If you go the a la carte route, you can buy a Power Groove from Buddies Pro Shop (
www.buddiesproshop.com) for around $50, plus $10-15 shipping. Now you have to get it drilled, which (depending on your location) should cost you somewhere around $30. Could be more if the driller actively discourages people from bringing in balls bought elsewhere.
So, I think the least you will get away with is $90-100. If you're not willing to spend that much, then just stay with what you've got.
BTW, on balls in that price range, you really won't save much, if any, by buying online. My advice to you is to talk to some of the better bowlers where you bowl, find out who drills their stuff, and go there.
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seadrive
Cogito ergo bowl