What about work in units of median annual household disposable income, which are at least somewhat responsive to the distribution of money?

What % do you think a reasonable voter should accept a person donating to a political campaign before it causes concern about the donor's influence vs the median household's voice?

Off the top of my head, I'd guess 500k USD is about 1000% / 10x median annual household disposable income in SE, which I think would give the median voter pause.

For what it's worth (my own view): I think about 10% (~5k USD) is obviously acceptable, and I expect most anyone would agree that donations at that level are fine. I think your proposed 1000% is obviously unacceptable, and I expect most people would agree with me on that as well.

I'm not sure exactly where the level is that opinion would flip, but I feel pretty confident about those boundaries.