IMO, you aren't spending enough. Holding back 3million is kinda pointless. It's not like you get interest on your savings.
At the beginning of the month, spend as much as you can but leave about 8-900k in case new tech becomes available for you to manufacture. Don't forget you can sell spare alien weapons (and you'll get TONS), alien armor and other alien crap you're not using (don't sell plasma grenades though).
Try to plant bases in the middle of a land mass. Like the centre of the US, S.America, Russia, Australia, that kind of stuff. Oh, don't forget the antartica too. Them alien bitches LOVE the Arctics.