It depends on the champ is why. IF your Jungle example is Sejuani, she doesn't usually get kills so if she is say 2/5/20 that is an amazing score on her.
If you are playing Kha zix, Lee, Vi, WW, Heca and other similar junglers, that is completely awful. You don't want assists on them. A 7/0 Kha Zix is terrifying, a 0/0/14 Kha Zix isn't scary in the slightest even though it should be roughly the same gold.
Supports are meant to only have assists since in a perfect world, the ADC should get all the kills. So technically a 0/0/20 support is the best possible outcome ever. Even if you are a kill support its still kinda bad if they have all the kills. A Brand that is 8/0 is scary sure, but when the ADC with him is 0/8 thats bad. He failed as a support. In reality she should have split the love if possible.
Assist are important but they aren't as valuable as kills, its the reason they are also worth less.
Personally how much money you should get should be based on how much you did for that kill. If you last hit kill stealed then you shouldn't get the full 300 since you didn't help out much, likewise if you are Raka and healed someone randomly to get an assist, it should be a low gold assist. That why it would reward people for putting a lot of work in. Say the support did 90% of the work for a kill, they should in theory get most of the gold for the kill and not the one that got the kill.