Long story short, any higher and it'll go into the negatives, due to how 4-byte integers work. Hopefully they'll get this fixed.

This does also mean you can't fully upgrade World Stealing Speed, since it goes beyond $2 billion.

I do not think this will be a problem in single player sessions. But may become a problem in multiplayer since you can go through maps quicker.

If in the event you do roll over into negative numbers, a fix would be to either go into New Game+
Or if you're the host or single player open .\GarrysMod\garrymod\sv.db in a SQLite editor

And then going and change "jazz_player_money" Earned and Spent stored values so that Earned - Spent ≥ 2,147,483,647. I recommend 2,000,000,000 to give some leeway.

But again, you're probably better off going New Game+, especailly if you're just playing on a server.

Edit: A demonstration on what happens https://youtu.be/06RZtVHWEBw