Just a few additions/corrections...
1. You can currently only buy countermeasures with Hawken credits, as far as I can tell. They're also available in packs of 10 for 1068HC, 50 for 5180HC, or 100 for 10146HC.
2. Functional internals are the same price as offensive and defensive.
3. Boosts are 1440MC for 30 days, 864MC for 14 days, 576MC for 7 days, 374MC for 3 days, and 288 for one day and double either your XP or HC gains. You can have both types active at the same time.
Unlocking a chassis and all possible items and internals for it costs 82023HC, or 158 hours of playing Deathmatch and winning every single game. There are nine unlockable mechs, meaning it would take at least 1422 hours to get everything.