they make their money on the cosmetics and impatient folks. Platinum is purchased currency and is used for speeding up constructions, unlocking things early, getting cosmetics, and opening extra slots for more weapons and frames. So the bulk of Warframes income is from that. On the flipside you can unlock everything in game without spending a cent via trades for certain premium things (prime weapons and parts or frames, and more.) with other warframers to unlock the various things that require plat.