Let's keep it short and sweet.
My girlfriend lives with her grandparents. I was told he'd be mad if I spent more than a few bucks (as in like 20) and already bought a $100 bracelet. If he gets upset or something (he's weird, some things he likes when I don't listen to him, others he gets mad about) what would a proper, gentlemanly response be? I'm sixteen, by the way.
"I chose a gift I thought your granddaughter would like and that I could afford, Sir."
However - Social taboos in play that the grandfather is reacting to, but can't articulate:
1. Expensive, ostentatious gifts
2. Jewelry as a first Christmas present, in a casual relationship
Both can (especially "in past generations") put the granddaughter in a tough position. They can imply you're trying to buy her. That the relationship is more serious than 16-year-olds should have. That your family is better than hers. That you're better than she is.
Consider, in my upper-middle-class family, full of adult professionals, very few gifts cost $100. I budget $100/person at Christmas, and only since I completed law school. My own family would be embarrassed - has been embarrassed - by gifts that expensive, even when there are no "How do they afford it?" issues. Expensive gifts from a teenager would have been rejected growing up.
People are just funny about presents. I often talk about how my in-laws give only cards at occasions (Father's Day, birthdays) when my family gives gifts and no cards. On AoM today was a story that could be interpreted as meaning a young person thought it was so important to give the boss a gift, she gave him stolen goods. My mother would be aghast to learn that I decided it wasn't worth the trouble to coach my new husband through giving me a gift. My husband is aghast at the inexpensive, unwanted gifts my mother puts so much effort into giving.
I know you don't mean any of those taboos I named. But they're part of our unwritten social fabric, or they were.
Stop using the word "like".
Make no mention, ever, to anyone about the dollar amount you spent on a gift. Ever. To anyone.
If asked directly what you spent on a gift, respond with "What I could afford without going over my holiday budget". If pressed beyond that, respond with "I don't believe that is really' appropriate, to discuss the price of a gift".
Your best response would depend on what he says. But the best demeanor, I think, would be detachment from the attack, and focusing on other things.
And that may be wrong. He may like a good fight. A joke may turn it. An excuse. "
But I'll go with Rebekah: if you're not, say, engaged, a $100 bracelet is a major statement that you aren't ready to make yet. Women take jewelry gifts seriously (so women tell me). Can you return it? Or save it for a time when the commitment is strong enough to merit it?