Do you have unlimited money, do you build things to sell, and do things really sell? :?: :?: :?: :?: ???
Well, it's just a matter of keeping things realistic. A nation with 5 million population isn't going to have £500 billion to throw around.
Here's a link that will calculate your budget:
http://www.pipian.com/stuffforchat/gdpcalc.php?nation=Myrth
Just replace my nation's name with your own.