I think it's not so much a matter of how much 1 ball would cost, it's more a matter of how much 500 balls will cost. I believe that is the minimum for a private run for Brunswick. A couple of Pro shops/internet dealers have had Big B run off 500 balls of several older balls (Fire Quantums, Green Quantums, Danger ZOnes, etc). Storm (whose magic minimum number I do not know) has created a few balls for a few Pro Shops also.
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Just like hand grenades and horse shoes, in bowling you only have to get close ...