If you're like me, you are busy building new toys to play with and working on that MR. After a bit, you'll notice that you are starting to lose your financial buffer and can't build things. No shame in this, we all go through it. There are a few parts in the game that either you can reach yourself or you can buddy up with someone else who has it unlocked. Lucky you!
Ceres - Gabii and SeiminiDark Sector Level 15 - 25 Tier 1
Survival or Defense
~25k+ credits/5 min
This is probably the first place that you can really stick it out with other people. Gabii is a survival mission while Seimini is a defense mission. Depending on what you are in the mood for, test out both and see what tickles your fancy. There are people who suggest extracting at the first available option due to the guaranteed 25k credits, then go in again. I tend to disagree MOSTLY due to the fact of having to reform a new group, restarting, etc. I try to aim for at least 20 minutes, mostly due to the fact orokin cells drop here - a lot. If your team wants to extract sooner rather than later, you probably want to extract with them. The enemies are pretty high level at that point and can really do a number on you. Remember, if you fail the mission, you get 0 reward.
Neptune - The IndexLevel 30 - 30
~75k to infinite credits
Easiest and fastest way to earn some credits. If you like to kill things, get a shard from them and take it to your goal, this is the place for you. In order to play this, you have to invest some credits initially, but the reward is pretty sweet.
Low Risk - 30k wager - 75k profit - 50 points
Medium Risk - 40k wager - 135k profit - 75 points
High Risk - 50k wager - 200k profit - 100 points
Now, you can either do it solo, where you get 3 NPC AI warframes fighting alongside you. The other option is to play it with other people. All are risky choices. There is a chance you either fail at beating the enemy team or you don't get enough credits in before the timer runs out. If this is your first time going in, I highly recommend you go with a friend who knows it or even try a low risk public game. It can be a bit overwhelming map-wise for the first time and I'd rather you didn't waste your precious credits.
One last note about this place. I highly recommend you bring a tanky warframe and some weapons that can dish out corrosive damage. It helps eat through the shields of the enemies and these guys do not play around. I have provided some images of what you can try on your weapons for this place. Bear in mind that these are for non-endgame builds, but you should have most of these mods available. There are always substitutions as well, given that they deal a similar output.
Eris - AkkadLevel 35 - 45
Dark Sector Defense
~ 30k credits/10 waves
This is an infested defense which is probably one of the better maps for small credit farming as well as nano spore gathering. It's a pretty easy defense as well, given that enemies rarely spawn behind you at the cryopod, with the occasional spawn on the sides. Just put on some tunes and let the numbers do the talking. Make sure to move about the map to gather the loot before extracting as well. Don't want to leave any credits behind!
SortiesLevel 50 - 100
Varies on Mission type
Varies on credits
Sorties are more towards the end game, but once you finish The War Within storyline, you can participate in them. All you gotta do is bring a level 30 warframe, make sure it can take some serious damage, and can output some serious damage. These missions also have a few differences from their normal counterparts, all depending on what the mission is. The bonus perk of completing these is that there is a final reward given upon completion of all 3 missions.
There are a few additional things to consider when farming for credits. A credit booster can help boost your return as well as a nice Secura Lecta. This weapon use to be amazing at the credit return until an adjustment was made. It still gives you additional credits, just not as much as it use to. If you've been playing for awhile, you might have obtained several repeats of warframe parts (Oberon and Harrow) as well as blueprints (Gorgon and Kavat Incubator Segment). These are also a great way to get some credits back in your wallet.
Ceres - Gabii
Neptune - The Index
Beginner's Guide to The Index