For me, it's less about credits per hour, and more about how reasonable the rewards are for the minimum investment of time and capital, combined with the risk involved. It breaks verisimilitude when I see a reward that makes me say, "You could buy your own ship for that!"
Baseline for me has always been trading, which to me means 1000 credits/ton/hop, as a reasonable "good" trade if you're traveling from one random station to another nearby random station. Yes, you can make a lot more while trading, but not from any old random station with an equally random location. Likewise, I consider a six minute investment in time to be a reasonable time investment for a single hop run. So 10,000 credits/hour/ton of cargo space is my baseline for me.
As for the maximum profit one should be able to reasonably make, that would be either 3000 credits/ton/hop, or a 5% Return on Investment on the minimum capital investment, whichever is higher. Or if you prefer, 30,000 credits/hour/ton or cargo space.
Needless to say, combat earnings should be higher than this, due to the potential risks involved, but again the rewards should match both the risks involved, and the minimum capital investment required to accomplish the mission.
The problem is that small ships have an almost absurdly high ROI given the capital involved, primarily so players will be willing to take risks and learn from their mistakes without being forced into the starting Sidewinder every time. Larger ships have a lower ROI, primarily because by the time you get into the larger ships, you shouldn't be making costly mistakes anymore, so your ROI can be lower as a result. Since most mission rewards need to be ship and pilot blind, a reward that is balanced for smaller ships and newer pilots is pocket change to larger ships and experienced pilots, while missions that that are balanced towards the latter group allows newer pilots to essentially skip over the "tutorial levels," and make their mistakes in ships they lack the skills to maintain.