Gold rewards scaling

I haven't played this game for a while. What I remember to have been my problem was the way the money rewards increased. I decided to play a slower playtrough and suddenly I recieved money rewards so high that I couldn't spend all my money. Do the rewards for quests increase with the amount of turns played or with the campaign progress? I would really prefer the later, as the game just became no challange with so much money (highest difficulty).