Someone running for parliament in the UK, for example, is allowed to spend about $0.26 per voter † in an average constituency (of 72200 voters).

It's certainly not a perfect system, but it does mean if you earn £100k it's very easy to become an MP's largest donor.

† (The rules are more complicated than that, of course - there's separate spending by the national party, one free mailing by the royal mail, rules for tracking the equivalent value of volunteered professional services, and so on...)