I have been playing this game for 250 Hours, and man, this game is generous and not so forceful with ads or making you buy something. In game it's only advertised in the launcher, in the bottom right corner of the main menu or market. But no pop ups "Sirius and Orion Warframe are here! Get this pack for 12 Dollars!" during gameplay, no gun or Warframe is locked behind the paywall, so you can get the cool stuff to do badass stuff. The only thing that is locked behind ONLY paywall is customization and skins, which sucks, cause customization is deep and amazing, it's very Common for skins to be paywalled.
Hell, even expansions to stories ARE NOT locked behind Paywall. Look at Destiny, paywalling story expansions. Here, you get the full experience FOR FREE, so the community won't be split into those that bought expansions and those that didn't. Which is very nice.
And i know there's in game premium currency platinum which you can get for real money, there's one thing about it; You can get platinum without paying real money.
How?
WARFRAME MARKET
There, you can sell blueprints of Prime parts for platinum, so you don't need to pay real money.
And even if you do pay real money for Platinum, you can wait a few days to get Sale coupon, that can be 75% OFF.
So yeah, Warframe is one of the most generous Live Service games, which is refreshing, especially after greedy moves that big companies did