Don't risk the Myst-N cases unless you're rich.
I personally have bought 40k nx for the first time this month.
Here's how much I can tell you I spent:
10,000 NX = Name Change
7,000 NX = GP Cases (I was bored)
2,000 NX = 1-Day weapons (1 M107CQ and 1 G36E Valkyrie in my Inbox)
2,000 NX = Black Hi-Sec keys (not for me)
7,000 NX Renewal Kits = Tried to renew my M416 CQB for Perm, I have 1 Renewal Kit in my Inbox as well.
If I was on a budget for NX
i would just buy the guns for 1 day and some renewal kits
Why?
If I was to buy a gun for 1 day and 1 renewal kit.
I would re-new it just to take the chance because you'll always have an extension.
I have about 12,000 NX now, and spent about 28,000 already.