Pretty good, and somehow getting better with time; especially considering how much you can get out of the game for completely free.

The game's main premium currency is platinum. There's no way to get it through in-game activities. You can buy it directly, it is included in most cash-only purchases, or you can trade it freely with other players. Most of the trading is organized on the third-party market board warframe.market, and the in-game trading chat... exists, I guess.

Most of the game's items (weapons, warframes, companions, upgrades) can be farmed through regular gameplay from random drops, from specific missions or boss fights, crafted from gathered resources, or bought using in-game currencies. You can buy most of them for platinum, but don't have to. The only payment-exclusive items are cosmetics (skins, helmets, color palettes), but not all of them, and inventory slots. There are also many late-game items that are impossible to buy and have to be earned. Some items are also sold in discounted packs. As of the latest major update (released literally a few hours ago), you get an additional discount for items of a pack that you already own.

The worst limiting factor for a new player is warframe and weapon slots. Your account can only hold a limited number of certain items, and slots are almost exclusively purchased with platinum (a small number can be earned through Nightwave, a free battle pass-like system). A new account starts with 50 non-tradable platinum -- my recommendation is to buy 2 weapon slot packs (12p for two slots, 24p total) and a warframe slot (20p for one).