Regiment Funds serve as the currency in Attack on Titan and can be somewhat difficult to come by and very easy to run out of. Nothing is free in the game, so if you want to develop new technologies, upgrade your existing gear, or obtain needed materials, you’re going to need some cash. There aren’t too many ways to actually earn Regiment Funds in the game, but you can increase how much you get if you’re skilled enough.
First off, you earn funds simply for the act of completing the various missions available to you in the game. This includes missions during Attack Mode, which serves as Attack on Titan’s campaign, Expedition Mode (multiplayer), and Challenge Mode. Even Survey Missions count, which are side activities you can complete to fill up your gallery and unlock other missions.
You’ll get rewards even for replaying a mission, so if you find one you’re particularly good at, continue to run through it for an easy grind. Do note, though, that later missions offer greater rewards, so you may not want to start grinding until you’re at least halfway through the game (there are five chapters).