Tax question: Imputed income of benefits [View all]

Is it taxed by the state or is it federal? What is a typical percentage rate it's taxed at?

The reason I ask is I have a friend who is legally able to enjoy a perk offered by my employer, however, for that perk I am taxed as it is added to my paycheck as imputed income. In this case, a couple of hundred dollars. I'd like to know because my friend wants to compensate me for what it might cost me. I don't want to pull a number out of thin air and overcharge her or conversely, get skint myself.

Thanks