Did I miss where the cost of the shottie ammo is listed in the book or the errata? I have went over both multiple times and have not found it, but I have also been known to be blind when looking for something specific as well.
I assume it's under "Rifles" in the Ammo Costs by Weapon Type table (SR6 p.262).
Incidentally, these are now mentioned in the Feb errata document.