Most F2P games create their own purchased currency, in Warframe it's "platinum," to help obfuscate to the purchaser exactly how much they are spending at any given time. Three weeks after buying X platinum for Y dollars, my mind likely won't analyze the purchase of something in a game as "I am paying the equivalent of $10 for a one-time use camouflage pattern..." No, instead to me it's "I just spent 50 spacebucks on tiger stripes for my armor!" Though this might have some questionable ethics, it does work...alas, in Warframe, I find it almost amusing, because at a glance, their prices are ridiculous (and not in the "Crazy Al's Used Cars, our prices are insanely low!" sort of way).
So at first blush, a new Warframe in Warframe (stay with me) is essentially a new class, and they cost somewhere between 75-375 Platinum (the newbie warframes are cheaper; the majority are 225+). Lemme pound that into my calculotron (at ~20 platinum per $1, depending on bundle) and that's ~18.75. Yes, that means almost $20 per class (albeit, you can earn these in game, lets not forget that).
"But Clockwork, World of Tanks sells tanks for more than that!"
Yes, but those tanks are all external from the progression and are more of an "addition" than part of the progression. Warframe's only "change" in class is by getting a new Warframe, so if you want to change class, you need a new frame...and some frames are a pain to get (I'm looking at you Nova and Nekros). Sure, the World of Tanks vehicles were even more expensive than Warframe.
That all said, Warframe has a unique way around this...once daily when you log in you get a "Login Reward" and one of the possible rewards is a 48 hour reduction in price of your next platinum purchase by up to 75. So that could theoretically reduce a warframe's cost down to ~$4.50. I actually think it's quite clever to keep the prices high for the "whales" (players with deep pockets who will spend regardless) while having the potential to drastically reduce prices to get players like me to spend a little on your game. It helps that everything in Warframe can be earned in game, with the exception of colors (as far as I know). In fact, the grind for most things actually feels fairly tame compared to some other free-to-play games. Whereas a new champion in league of legends can take upwards of 20 games (times ~20 minutes per game..), a new warframe can theoretically be obtained in 3 runs of a mission, plus 3.5 days of crafting time.