Jagex uses "ints", which can only handle numbers ranging from -2,147,483,648 to 2,147,483,647.
If Jagex ever switches to the socalled longs or even floats or doubles, we're gonna deal with a hell lotta bugs and crashreports the first couple of weeks. The very very max number that Java can handle up to date is using doubles, which gives you the possibility to get up to 1.79769313486231570E+308 GP, which ofcourse is an unthinkable pile of money that even 500 Chessys can never, ever get together.
But in general, nobody gets or needs the current 2.147B max cash for anything but showing off. No way Jagex will ever take a big risk for the 10 people that care about the current max amount of cash. It's useless and therefor stupid to implent it at this moment.
On the other hand, I believe Jagex could ever come in a position where they'll be forced to switch all of their "ints" to "longs". All players get richer every day, so how long will it take that 100M will be considered as "not that much"? If they ever need to switch their max, I'm sure they're very capable to do so. I mean, they even have their own language inside Java, called RuneScript, for their staff that don't know anything about Javascript.