Dave,
This is one of the more thought-provoking BBQ questions I've read in a long time!
I would think that the best answer is to bring the error to the attention of the butcher so it can be corrected. You get the product you want at the posted price, and the butcher learns from his or her mistake and doesn't suffer an economic loss. Next time you visit, the butcher may remember you as "the honest guy" and provide you with better service or some other perk.
Declining to buy the product would also be acceptable, but wouldn't help the butcher learn from his or her mistake.
In the end, perhaps "don't do to others what you wouldn't want them to do to you" is the best approach.
Regarding BBs, here in N. California they're $3.49/lb for 3 slabs in Cryovac at Costco and $3.10/lb in individually wrapped Cryovac at Smart & Final. Grocery store prices are closer to $4.99/lb unless there's a sale.
Regards,
Chris