Basically, it's a service charge for buying the ball elsewhere. If the pro shop sells the ball and makes $70, he wants to make the same if you buy it somewhere else.
Blank drill prices used to be lower, but with the internet, many shops felt like they were being forced to take a pay cut when people bought balls online, so the price went up. Plug prices haven't gone up because shops didn't feel cheated in the same way.
Plug prices should be higher, I agree. But, then people would come to the forum and complain about being overcharged for plugwork that used to cost much less.