Jeez.
I spent $10 on platinum which I used for more warframe and weapon slots. That's all you really need.
This is all you'll ever really want to do in the game microtransaction-wise, and only in the long run.
Basically, the game is designed so that you only get Mastery Rank progress (which is basically an account level) from gaining experience with gear that's not already maxed out (or hasn't been maxed out by you previously). As a result, you end up going through a lot of weapons, frames and sentinels.
You'll only actually "need" to spend Platinum on extra slots to hold them if you just don't want to get rid of your old stuff - you'll need maxed-out gear to do endgame content, and some weapons are just better than others, and some are better for certain situations (i.e. the starting MK-1 gear is intentionally underpowered, because it's starting gear - so everything's at least a bit of an upgrade from that).
Considering how many hours I've put into the game on Steam (I think I'm nearing the 400-hour mark...) I'm pretty happy with how little I've spent on it - most of my platinum has gone toward extra slots for weapons and frames, but I'm also pretty happy with the cosmetics I've purchased... Since color pickers can be used on anything, and scarves on ninjas gives a very PS2 Shinobi vibe.
Probably not going to go hard into this version until possibly after an account transfer is set up (or account sync, but that's more of a pipe dream at this point).