@spyder335 The only thing you can buy with cash is the gun itself. Certs are not purchasable outside of buying xp boots and gaining certs per the increased xp rate.
I've put in well over 200 hours in the game and to date have put in about $30 of Station Cash, and 3 months of premium membership (which gives you 500 SC per month). I've unlocked all the air guns I want, all the vehicle guns I want, all my MAX unlocks, and most of the infantry guns I want. I still have almost 1400 SC remaining. The key is to only buy things when they go on sale as pretty much everything will go on sale and is always 50% off.
As far as "Are the upgrades worth it?" If you're talking about guns, it all depends on your play style as all guns are side grades and have trade-offs that may work better with one player than another (clip size vs rate of fire vs recoil vs damage output vs range). If you're talking about the cert cost to upgrade a gun, unequivocally yes. Certs will allow you to stabilize recoil, put a better sight, change ammo types, add a grenade launcher, add a shotgun to the gun and really make it multipurpose.