There are many factors at play, as has been pointed out, but the base factor is your reputation. The population, economy type, and System Faction States all play a role. But your reputation acts as a multiplier against the base payout.
Indeed. It isn't possible to 'game' the mission board as such just by building reputation, there's too many influential factors in play tied to the BGS for that to be possible.
Won't go into where it is, but a day or two ago found myself in a anarchy system where looking at the traffic report it sees about 5 players a day in terms of traffic just jumping through and looking at the bounty hand-ins there and nearby I'm the only one working in the area. There's a planetary base with a NPC group that each time I've logged-in to play offers anywhere from 8-13 combat missions to hunt pirates from various near-system factions, and some skimmers assault missions. Most pay a minimum of 150k up to 450k at neutral reputation and offer a nice set of materials and cargo almost every other mission.... and that's all they've ever offered.
Problem is I think I stumbled on a slight bug with how the new mission system behaves with a 'legal' corporate faction (Federation affiliated) handing out the mission in a anarchy system. It treats all 'kill xxx of yyy faction' targets as 'pirates', regardless to if the target system is actually a well secured system and the faction they selected is a legitimate and above board group.
Frequently I've been sent on missions to do something like kill "6 pilots of the <system name> Jet corp pirates", only to arrive there and find that the faction is actually a policed system and the faction is a federation aligned corporation that holds main influence over the system... As you can probably imagine you don't exactly find many pirates belonging to a above board faction that's the main authority in system. Not even in haz res sites.
So now before I take any of their 'kill xxxx pirates' missions I always check out the system and target faction to see if there's a chance for the game to even spawn pirates of that faction before I accept the contract.