It's more about the aid slots than the actual monetary amount nations are sitting on. 3 million is pretty much spare change for me, but I can only send out 18 million every 10 days. During that 10 days, I wouldn't be able do any tech deals.
Assuming I'm sitting on a billion, it would take me 56 full aid cycles (560 days) to run out of money, if that was all I was doing. And that's not counting additional tax collections of course. So a theoretical 100k nation, and nations much smaller than that as well, that is not at war and is only sending out aid will never run out of money. It what those other aid slots could be doing that count.
There's my monthly allotment of econ.