how to afford rank 4 vehicles?

I've researched my first rank 4 plane and there was such a sudden jump in cost, it was expensive af (it cost me like over 200k out of my total 300k i believe, including crew training), even tho i was efficient with my SL expenses. Soon I'm gonna finish researching yet another plane from the 4th rank, but if i buy it then I'm gonna have barely any SL left and the only way out will be to grind with my previously unlocked planes for what seems like eternity.

i guess this is the moment when you either pay with ur sanity or ur real money, huh?