As with all things it depends on who you know and who does the job.
Mags are quite simple bits of kit ... they need the points gapped & timed and bearings lubricated. They need cleaning and checking for cracks or damage. Not much more difficult than the distributor on a 1960s car.
My two Bendix mags were overhauled for £100 each at the last 500 hour check. They went with a batch of mags sent from my local flying school to a well known aviation repair and overhaul company in the west country.
Don't even think of trying that as an individual owner ...
I took my mags off and refitted them myself using a simple electronic timer obtainable from LAS. Can't remember if that was a permitted task under annex VIII, but who cares when such ludicrous amounts of rip-off money are involved.