Anyone that tells you they don't know the price is playing dumb or is dumb. All you need to know is your state's sale tax and do the math.
Sometimes you'll get hit with a small city or county tax in an area you don't know (usually 1-2%), but these are rare and in the most populus areas. Most places it's just the state tax.
Point is if you are shopping in your home state in your home city you know exactly what you need to pay. Doing the math is inconvenient, I won't argue that. But it's not a total mystery. If someone were that concerned they can lookup tax rates and calculate.
What dollar general is doing is they are updating a price in their computer system, but not changing the tag. So an item with a $5 price tage should cost $5 plus the local tax. But in reality the item was $6 plus the local tax because dollar general didn't change the price tag so you can't know the price.
Don't get me wrong I wish everything was included on the tag, but it's not some unknowable random price. There are rules