“I don’t need your two cents,” probably carried a lot more weight when the penny had a higher value.

maybe, but we're talking about pennies, which even back whenever were still the smallest unit of currency.

i guess the expression always came across to me as modest and self-effacing, or maybe even collaborative (e.g. i add my small contribution, if we all do the same maybe we'll have a collection that's worth something when all the parts are counted together)