Did l read correctly that 2.3 will decouple the mission/passenger boards, so there should be a greater allocation per? Right now it's about 20 per mission/passenger board, and positive/negative BGS states tend to get more of the allocation than neutral states.
Anyway - OP your question needs more detail. What do you consider a "valuable" mission? Are you looking for specific types of missions, or just overall mission credits? If the answer is the latter, then the best thing you can do is rep up with ever faction in a system. The more rep you have with them, the more you will be able to pick the best paying missions for yourself. I was running simple haulage missions yesterday that ran from the 500k - 1.2M Cr range.
Also, bulk passengers are a good way to make cash, especially if you don't mind playing dirty. I got a shady redirect yesterday that turned a simple 400k passenger mission into a 2.5M passenger mission.... I mean, passengers require lots of paperwork. There are bound to be mix-ups... from time to time :|