Write them a thank-you note just as if their gift cost $100 and forget it. It’s not your place to look in other people’s wallets. It’s possible that $20 is all they could afford.
While wedding gifts are expected at a wedding there’s no rule as to how much a guest must spend, it’s at their discretion. And while some will say that a gift should equal the cost of what the couple spends per person there’s no way to figure that because it would include a total of the food, invitation, favor, program, wedding cake, flowers on the table, music, etc…