Diminishing returns for missions

[Sandermatt]

Currently the economic aspect becomes too easy as the game progresses. I think it would make sense to reduce the rewards over time that one can get from a certain region. So doing tasks in kerbins orbit gives a certain amount of money, but from there on gradually decreases down to a certain floor value (let's say 100% (no decrease) on easy and 20% floor on hard). This would make the later game more interessting.