Well, the way this seems to work is sort of strange. It kind of makes me wish that I hadn't ascended my Soothsayer and Valkyrie. The bigger the difference in Power Rating from their team relative to yours, the more gold they are worth (or so it seems?). If this is the case, you work towards making stronger teams to take down tougher enemies, but only close the gap and get relatively the same rewards you would have, had you not ascended/leveled/traited the troops.
I'm not 100% if that's how it works, but it appears that way. It is further supported by seeing @Tacet 8K+ teams going against nearly anyone else with a 8K+ Ulted team not giving him nearly as much gold as I would get for fighting the same teams. Although, it may be affected by player level. If the former is accurate, it would be pretty counter-intuitive, but what do I know...
It also may be worth mentioning that I have had fights where the opposite team was worth 1,900+ gold, even over 2K. I will screen-cap next time, as I wasn't aware of this thread's existence.