Really depends on if the pro shop you go to is a hobby and/or part time gig or a business for the owner. Most people who do it for fun or do it on the side don't generally need to generate the revenue that an actual pro shop business needs to. Sounds like you go to a pro shop business, in which case, yes, that's the going rate, and is a fair price regardless of what anyone else says. Considering the suggested retail of a ball like the Sure Lock is $299.95 BEFORE drilling, getting it for significantly less including the drilling is a good deal. Pro shops really can't afford to operate on the margins they're forced to by hobbyists and the internet companies, which is why you don't see many good ones around. Internet companies make their money on the dozens of advertisements and banners on the site, they really only sell the equipment so they can make money on the advertisements, absolutely zero way they could function or operate at those prices if sales were their only source of income.
A shop that can afford to offer quality at a cheaper price than that and is actually ok putting in that kind of effort for that little return is truly rare, and you should definitely patronize them. There are a lot of benefits of purchasing through a pro shop that definitely make it worth the few extra bucks. Obviously I'm biased LOL, but I can also make numerous points and defend them all in favor of purchasing at a shop rather than online, even if the shop is more expensive IF IF IF that shop has service and quality worthy of the extra bit of cost. Pro shops should cost a bit more than you can get it for online, but they should also make it worth the cost.