The cost per hole will vary greatly, depending upon location and local gunsmithing demand, from a low of $10 to upwards of $25. - meaning that, unless you're going to ship it out, you'd better shop around locally, or at least within a reasonable traveling distance.
Think about how a $10 x 4 quote that's far enough away to spend another $10 on fuel for the trip, plus other wear/tear expenses, would put that $20/hole quote from next door in perspective.
Besides which - it's not about how inexpensively the job can be done, IMHO it's how well it can be done, i.e. w/o cratered hole edges, or misdrilling, etc, etc.
I now do my own, but before I did I always first looked at the results of the work of whoever I was thinking of having do the job.
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