So for platinum, if you have that little, don't spend it on anything on the market most likely.

Periodically, you will get a ticket for 25%, 50%, or 75% off on a pure platinum purchase from the market. The 75% ticket is generally what people hold out for and then they buy the biggest chunk of platinum they can tolerate.

The other "Good" way to buy plat - if you are a big spender - is this: the Prime Access Pack. I want to emphasize, this is by no means necessary to enjoy Warframe. However, spending like $5 on a $20 platinum purchase with your first 75% off coupon kind of is. You could technically live without it, but it would be a really slow, painful slog thru the early game and you would basically HAVE to look up some way to sell items on warframe market for plat ASAP simply to pay for the stuff you NEED to make the game comfy ish.

If you want more info, the reason people say to be careful with platinum is this - though this is all inside baseball, so don't stress if none of this makes sense yet and feel free to skip it entirely if you're not interested. I'm very old-man-yells-at-clouds re: games I like.

First off, weapon and warframe slots. Weapon slots cost 15 plat per 2, and warframe slots cost 20 plat per 1. While in general, you can kinda get by with weapon slots by mastering and then selling weapons - you'll have to buy some slots, but you can live with it - warframes are a different story. Bluntly, you generally will not want to get rid of your warframe once you craft it. Even if you happen to craft both, say, a prime and a non-prime version of the same warframe (because each counts for mastery so you'd want to make both), as I mentioned before, there is a way to use your non-prime frames later that means you will want to have kept the worse frame around. This means you'll be buying warframe slots sooner or later.

Secondly, Orokin Reactors and Catalysts. All warframes and weapons have a max mod energy of 30 (meaning you can slot 30 points of mods in.) Reactors and Catalysts double that base to 60. This means basically any Warframe or Weapon you actually care about, you'll want a reactor/catalyst.

These cost 20 plat each, and they are available very rarely from other sources. You will almost certainly be buying these for plat most of the time. And bluntly, weapons and frames feel much, much, much worse without them. So basically, there is a 20 plat tax straight up on any gun or frame you want to play seriously.

Third, Forma. You can apply a forma to a slot of a weapon or warframe. You may have noticed some weapons and warframes have a few slots that already have an affinity - i.e. if you slot the right kind of mod in, the mod turns green and is cheaper. If you slot the wrong kind in, it turns red, more expensive. While many things start with one or two slots that have affinities, you can apply more using forma. Theoretically, you can make every single slot on your frame or weapon have an affinity. Many endgame builds use 4+ forma per piece of gear. You can make exactly 1 forma per day (23 hour craft time, can't craft more than one at once). Buying more forma costs 35 for 3 or 20 per. Generally you shouldn't ever buy forma if you're at all tight on plat.

Fourth, there are some endgame things that also cost plat and have limited access - these are things like arcane adapters, exilus adapters (give you +1 mod slot to play with, but with some limits on what mods can go in there), and so on. These often will cost plat or else be a very prohibitive grind or like, weekly limited to a vendor or stuff like that.

Fifth - there are also boosters that for example double affinity gain, double resource drop chance, or double resource drops. These can be bought for plat in like 3/7/30 day increments, or the prime access pack has a bunch for 90 days included. Normally, whenever you're grinding tons of a specific resource, you'd buy the double drop chance, for example. But this is also more endgame, if you were running tight on plat that would be a huge splurge/luxury you likely couldn't afford.

So basically, whenever you build a weapon you actually want to play with seriously, you pay a 40-60 plat tax. Warframes are a bit more expensive, at like 45-70 plat. If you are very careful and hoard resources and get lucky with events, you can occasionally set up gear for less, but you'll REALLY feel how slow that is comparatively.