yea, i'm not sure if i agree. that might be true for credits, but definitely not for mats since there are missions that offer loads of money but no mats.
The raw credit reward of a mission doesn't influence whether or not materials are one of the reward options, only the grade and quantity. Influence and reputation are both 300-500k per + (in addition to the base rep/inf rewards offered in the credit rewart), Materials are (I forget the rough figures I drew up last time for all grades), and cargo is the value of the cargo per galaxy average.
It's a direct exchange of potential credit reward to the respective items. For missions that offer loads of money,
if there's a material reward, it won't take away more than around 2-2.5m off the reward for 5 x G5 mat rewards. Corollary: You'll never see anything like Courier missions ever offer G5 mats, unless it's to a distant station (which ramps up the potential credit reward).
For example, below's a 3x G5 mats, which virtually blats the 1.25m credit reward (because 3 x G5 mats is worth roughly half that 2-2.5m I quote)
As a comparison, 5 x G4 materials is about 1.25m credits, since they're worth roughly 100-175k
Reputation affects your raw reward total, so usually if you have lower rep you'll see less high-grade material offers, because the raw credit offer is lower. But per below, for large-credit missions, you can still see 5xG5 mats even if you have low rep.
The below mission is offered by a faction I'm unfriendly to, and is a wing mission to ship 1600 units of cobalt. Note, 5 x G5 deducts about 2m again. This correcrts my previous estimate of 200-300k, putting them at 400k a piece.
As the final contrast, here's a mission paying out 16m... but note the 5xG5 mats still deduct only about 2m off the credit reward, maintaining that "exchange rate"
tl;dr if you've only got 1m or so to play with, that will only ever offer at-most 2 x G5 mats, and more likely variations of lower grade mats. I'm unsure what the going rate for passenger missions is, but more passengers should = more credits, which means greater chance of having G5 mats as a reward option,
when mats are offered as a reward.