Michelle is right.
For a "pro" to get a ball off the truck is $50-$100, then $30 for drilling - grips and slugs? Add another $10-$20. Granted - not what you pay at your local center, but when you may need to drill up 3-4 a week just to stay in the hunt - NOT CHEAP!!!!
Second thought - remember the big flap over the space shuttle toilet seats a few years ago? The government was being billed $700 apiece for them? Well - simple economics. When you manufacture a product, you have to AT LEAST cover your R & D and production costs - that's to just break even. When you make 10 toilet seats over the life of the project, they're gonna be expensive...
So when you're talking bowling balls - I don't know how many of each model are made, but I'm sure that each one isn't in the 100's of millions... They're gonna be a bit more expensive than golf balls - which I'm sure are made in at least the 10s of millions for each design.
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Bowling? Of course it's a sport.