I can't find any info on the algorithm for calculating credits earned in a ME3 multiplayer match; namely, whether or not awards or individual score affect it. I noticed that you gain credits for completing the "special" waves, but it seems like there's more to it than that.
Your main source of credits comes from completing objectives in a fairly speedy way. Jumping to a higher match but taking way too long to complete it doesn't help you.
Note ... while there is certainly a variable amount of credits added based on how long it takes you to complete the objectives, I mean that if you can consistently plow through silver without dying you can make more than struggling your way through gold.
Bronze Match – These are the easiest matches, completing one will earn you 15,000 credits depending on how quickly you complete the objectives
Silver Match – These are the middle tier matches, completing these will earn you around 30,000 credits
Gold Match – These are the hardest matches. You’ll probably want a team of good players, higher level characters, and better weapons unlocked for these. These will net you 60-70,000 credits.
Wave 3, 6 and 10 are objective rounds and upon completing the objective you'll earn credits. In gold, you'll usually have around 30,000 by completing wave 6, and 70,000 by completing wave 10. Note that if you die during wave 10 but you completed the objective, you'll still earn as many credits as is possible from the match.
The special missions on wave 3/6/10 have set reward values, based on the challenge level (bronze/silver/gold). There is a bonus for completing the objective faster - the more time you have left, the higher the bonus. Unfortunately, I do not know the exact calculation, but it's not a lot of credits.
With the upcoming Retaliation DLC and Patch 4 Update, you will earn additional credits for successfully extracting at the end of a match.
This theoretically means you can get:
Note that the 10% credit bonus only applies if you yourself is alive at the extraction point when the timer runs out.