I think a possible guide, in this case, is to give enough to cover the meals at a reception (if you're going), plus at least half more. Obviously not all receptions are as fancy as others. If you think it would equate to a $100 restaurant bill, I'd at least give the couple $150. If you're not attending a reception or it's a cheap beer, fried chicken and biscuits one, less.

If you truly can't afford more than $100, than it's fine.