The other responders are correct, you should know to not automatically rearm your mech right now, allow it to fill for free up to 75% of your ammo.
Just wait til you can go out and farm Artemis ammo from NPCs when they implement that part of the game...just don't hold your breath, okay? 8-)
Edited by Relaed, 01 December 2012 - 07:47 PM.