Because Bethesda still kept some level scaling which I hate, it means both the quests and rewards adjust to what level you are at the time.
Here, the "leveled reward list". The gold you get has nothing to do with how much money the guy had, but depending on your level. There are 4 grades of gold rewards for each level bracket.
I would prefer if the quests are fixed, which means the amount of gold reward would also be fixed. But that's too much work for Bethesda. Level scaling means less work balancing the monsters and treasures.
Also note that it is impossible to get 2000 Septum from delivering the package to the Greybeards. $1800 if you are level 40.
Realism is not important enough to break the game for. The whole idea is to make sure you are not rewarded pocket change.