as far as I can tell there's not a big payoff until you reach the higher levels and start collecting the gold. this heavily punishes the casual player.
Blizzard should take most of the gold from the level 50+ range and distribute it evenly throughout the lower levels. for example reduce the gold gained from 150 to just 50 once level 50 is reached, then feed the difference into the lower levels.
level 50+ should be more of a soft reward with diminishing returns rather than the main goal.