I'm currently in the process of creating a spreadsheet with the average credit payouts from the Score Attack playlist in firefight matchmaking. Here is a list of each individual payout type and an approximation of its size if 2x Score Attack is played.
Credit payout given for every full minute played (in a game) and will increase by 10% per level after Field Marshall. Here are the payouts for a 10 minute game for few different levels.
This can be compared to the none matchmaking credit payout for minutes played. However, this payout is capped to the 10 min payout when the game exceeds 10 min. Here is this payout for the same levels compared to above but please note estimate values has been used for Field Marshall and Here which means that the actually payout may differ slightly.
With these numbers, we can find that the Score Attack Matchmaking payout for time played is approximately 180% high than for none matchmaking gametypes. However, there do also exist few playlists like Grifball and Firefight which has a slightly higher credit payout per minute played.
Credit payout for making actions which contribute towards commendation progress or when reaching commendation milestones. The milestone payout is insubstantial in terms of size so the progress credits will be shown for the Firefight commendations. However, please note that the commendation progress payout also be payed when the commendation is maxed with the exception of Campaign commendation progress payout which will only be payed for commendations below Silver.
For me, the average commendation credit payout for a 2x Score Attack games lies around 800-900 cR/game but this number may differ due to a different map or playstyle.
The third and final credit payout is the Slot Machine which is a randomized credit payout. This payout will be given for every completed (no quitting) matchmaking game that is longer than 2 minutes. Based on the Slot Machine payout for 608 recorded games, the following numbers got found.
If the 3000 cR payout is ignored, the maximum payout is 999 cR and the expected payout got lowered to 377 cR. Finally, out of the 608 games, 37 gave a slot machine payout of 3000 which is also by far the most common specific slot machine payout.
In conclusion, the credit payout in Halo Reach is based on a multitude of different factors and with careful testing, the expected payout can be specified.