how much is an acceptable mark up for service parts?

[Perfectlap]

So I got a very good quote for routine service from a local shop. While looking at the parts list I noticed they were charging over $100 for spark plugs. All other parts were within $10 of the Pap Parts.com prices. I pointed out to them via email that the plugs are available for under $10 each and provided the link.
Which brings me to my general question, I understand shops make some money on a marginal mark up but what is too much?