In previous versions SI has used 32 bit integers as the data type for storing the club balance and transfer budget. I’m pretty sure that is still the case in FM2008. That means that the highest number you can use for any of these is 2^31-1, which is 2,147,483,647. So, yeah pretty much 2 billion.
I would probably recommend using lower values like 1.5 billions to be on the safe side, because if you use the maximum number, and then actually earn more money then you’re in trouble. The reason for that being that the funny thing in programming is that when using 32 bit integers, then 2,147,438,647+1 does not equal 2,147,438,648, but -2,147,438,648. So, if you ever go above the “magic” number, then you’ll be in serious debt. Theoretically it should work the other way around as well, so if you can manage to get a debt of a bit more than -2.1 billions, then you’re very close to becoming a billionaire.
I just wish real life would work like that as well