Rich the Kid has apparently decided New York City's economy needs a shot in the arm, because dude boarded a plane bound for the Big Apple with $100k cash!!!

We got Rich Sunday at 5:30 AM at LAX and he flaunted his stacks for TMZ. Rich is on his way to New York Fashion Week ... he was mum on what he's gonna do with the stash.

And, check this out ... we did a little digging, and there's no limit to how much cash you can carry on a plane, but TSA can ask questions to make sure you're not up to no good.

Fact is ... the cash pales in comparison to the rapper's watch, which he says is worth $120k.

And, speaking of Rich showing off his riches, he might want to think about roaming the airport with all that loot, looking for snacks without his bodyguard anywhere in sight. He was robbed at gunpoint back in June by several guys who broke into his apartment.

Rich and his GF, Tori Brixx, were injured -- she was pistol whipped -- and both ended up in the hospital.