In Finland, there is a peculiar system (päiväsakko/dagsböter/day-fines) where drivers who commit violations on the road (such as speeding) are punished with a fine proportional to their income.
Does this system also apply to foreign non-resident drivers? Or do they pay some fixed amount since it's hard to determine what they earn abroad?
You should consult the SpeedingEurope page to see the fines laid out in per-day amounts, where the lowest is 6 EUR / day. The police will work with that amount with a tourist, see this Reddit post:
The law says you're supposed to report your real income and the fine is based on that. In practise you'll get the minimum fine since there is no way to check if you're lying or not. Woudn't be suprised if they don't even bother asking anything. A year or two ago some Finnish NHL player said to police that he's not earning anything and recieved minimum fine. The police later figured out who he really was and then charged him for lying. So it is possible to get caught and punished, but this probably doesn't affect your average tourist.
