If we wish to obtain Credits in Halo Reach, it's essential to know how Credits are given to the player. This in turn makes it possible to find some of the most effective methods to obtain Credits in Halo Reach.
First and foremost, credits are given based on different actions and this includes Time Played (Completion), Commendations, Player Performance, Challenges and the Slot Machine. Out of these, the Time Played can be split up into 3 subcategories none-matchmaking, matchmaking low payout and matchmaking high payout.
Section 1.1: None-matchmaking time played
The Credits given from time played outside matchmaking will be given by a constant amount for every minute played and this payout is capped for 10 minutes. This means that you will get the maximum payout after you have played for 10 minutes. This payout can also be increased by ranking up and after the rank Field Marshall, this payout will increase by 10% each time you rank up. Here is a table of time payout for time played at each rank and the payouts written in Italics
Due to these relatively low payouts, it's not recommended to obtain Credits if you don't have access to matchmaking and that the gamertag is a Xbox Live gamertag.
Section 1.2: Matchmaking time played
In some matchmaking playlists like Score Attack and Campaign, the player gets a low Credit payout for every minute played. This payout unlike the none-matchmaking payout is not capped at 10. However, there do also exist some form of cap for this payout so you cannot AFK in Campaign to obtain a large amount of Credits but it's similar to the none-matchmaking payout that in terms that it increases by 10% when you rank up after the rank of Field Marshall. ere is a table of time payout for time played at each rank and the payouts written in Italics
However, it appears to follow the same rule as above that it increases by 10% per level after Field Marshall and it appears to be dubble the payout compared to playlists with a lower payout. Some playlists which has the higher payout are the Firefight and Grifball playlist. Here is table with estimated time based Credit payouts for playlists with higher payouts.Section 2: Slot Machine
In Matchmaking, you will also get a random amount of Credits if you don't quit in the middle of the game and that the game lasts longer than 2 minutes. The size of the slot machine can be split into 3 categories.
Out of 933 games played, the following proportion of the payouts fell into each category.
When the payout rolls in Category 1 and 2, the distribution of the different payouts can be estimated to be uniform (same probability for every possible payout). The expected payout from each category can be seen below based on the same sample of 933 games.
Please note that there is a slight underrepresentation of payouts in Category 1 and a slight overrepresentation of payouts in Category 3 due to human factors. Due to this discrepancy, it's highly likely that the probability that a payout is in Category 1 is 75%, Category 2 is 20% and Category 3 is 5%. If these numbers are used and it's assumed that it's a uniform distribution in Category 1 and 2, the expected Slot Machine Payout is 506 cR.
Section 3: Commendations
The amount of Credits given from Commendations can be split up between Credits from progressing Commendations and reaching Commendation milestones (Iron, Bronze, Silver Gold, Onyx). However, the Campaign Commendations will only give progression Credits when they are below Silver which is a big contrast to Firefight and Multiplayer Commendations which will always give progression Credits (even when maxed). In 2x Score Attack, I do usually get around 920 Credits from Commendation progression per game and it's a similar number for the other Score Attack (playlist) gametypes.
Section 4: Challenges
In Halo Reach, there exist the possibility of getting Credits from Daily and Weekly Challenges. However, I haven't done a detailed analysis about the size of the Challenge payouts but the Daily Challenges appears to usually give around 2,000 and 3,000 Credits upon completions. However, due to only 4 Daily Challenges, Challenges are only a small contributor to Credits except if you don't have access to Matchmaking or if you are at the earlier ranks when the Credits for time played is much lower.
Section 5: Player Performance
Haven't analysed Credit payouts in this Category.
EDIT [25/5-2020]: This number appears to be strictly correlated with the Completion bonus. It's highly probable that this is a 20% bonus to the Credits given from completion.
Added an approximate factor of conversion between playlists that has a higher Credit payouts and lower Credit payout in Matchmaking.