What engine?
How many miles?
Has the maintenance been kept up to date?
The price was fair. Over the internet you’re paying for only the part. In the shop you’re paying for their expertise, use of their tools, their administrative costs, and use of their bay as well as the part. $120 actually sounds like the “minimum charge” that most shops use. They have to get something fair to tie up their assets for an hour.
Replacing the coil is not just treating the symptoms. Coils fail from age and exposure to underhood temperatures. There’s a lengthy technical explanation involving differences in the coefficient of thermal expansion of the copper wire and the varnish-like polyimide-amide coating on the wires and the loss of elasticity of the coating…oh, never mind. Coils just fail without the failure having been induced by some other factor. It happens. However, they have a fairly consistant lifespan, and if one failed the others may begin to fail one by one.
If it were my vehicle, I’d just change the rest of them, throw in some new plugs, and I’d bet it’d run great for years to come. Unless, of course, it’s burning oil and fouling plugs or something like that.