How much should a drummer or percussionist be paid to add some drum sounds to a song in a studio session if it would only take them about an hour at most?

It depends on whether you’re hiring an actual session pro, or not.
A professional session player will want at least Union Scale, which is currently $100/hr. (in CA, NY, and TN; it varies from state to state) with a three hour minimum (meaning, even if someone “nails it” in 20 minutes, they still get $300).

A “guy off the street” will work for considerably less – (maybe even for an album credit or points on the sale of the record) – but (probably) won’t be a “one-take” wonder.