Fallout – What’s the biggest caps reward from Mysterious Stranger

There are three factors.

Luck, luck, and luck.

Luck determines how many caps wasteland explorers get, and how many caps you get for successfully rushing a room's production. I believe, however, that the caps you get from leveling up a dweller is not affected -- they provide caps equal to their new level. Reach level 4? That's 4 caps. Obviously if you have 200 level 48 dwellers, you'll be raking in the dough soon enough.

You only get bonus caps for leveling up when the dwellers actually level up.

There are several other sources of caps (exploring the wasteland, selling gear, lunchboxes, bonus caps when rooms finish, etc) but once a dweller is level 50, you won't get any further level bonuses from them.