I've never had this "exact" thing happen--people tell me I undervalue my work at the most, actually. But, hypothetically, if it were to happen, I'd be pretty upset. But for a week at the most. I'd remember then that I am in fact making a wage off my art, so it's gotta be worth the pennies people are paying for it.
For me, it's not only the value, but the quality of work an artist can produce. Surely you're not going to pay $100 for a piece of napkin art, but for something refined on a piece of 11x17 bristol board inked in india ink? Hellya.
I guess it all depends on the artist, right? But if you're a seasoned artist, drawn for most of your life, and people like your work, it should be fair to charge a bit more than what a high schooler would charge, right?