WHat you should pay is very regionally specific, according to the costs associated with such activity. No one here on ballreviews, except those in the area in which you live should give you specific dollar amounts. If you tell your pro shop you'll be doing 5 balls or 10 balls, then you should probably get a volume discount. Plugging, the routing, then sanding, then drilling, then customizing the hole is not 10 seconds work. While nothing should go wrong, it is too easy for something to go wrong. Some, like those with inserts are easier and quicker than others.

You should get all the balls that you are going to use done; so that, in as far as humanly possible, they all feel identical.
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Bowling: Just like hand grenades and horse shoes, you only have to get close.
Life: Deal with what is.