To me, it all depends on the canon. I know the MW3 MFBs repaired WAY too fast (for gameplay reasons), but I don't know how quickly ammunition can be replenished. I also think it should tie into the campaign and the availability of logistical support to your company.
Let's not forget that leaving the front line to resupply is a LOT of time you're spending, basically as dead weight on your lance, especially if you're in a 4/6 plodder (or worse). Also, going back to resupply, alone, makes you ripe pickings for being taken out by roaming scouts.
I think that it would add a lot of interesting tactical decisions, but like I said, I would only be accepting towards canonical reloading times, I imagine that a trained and already-prepared reload crew would work much like a pit stop in car-racing, just with bullets and missiles instead of tired and fuel.
EDIT: As for cost, it could be tacked on to your repair cost at the end of the mission, or simply automatically deducted from your mission's earnings. I think I like that last one better.
EDIT 2: It should also cost more than ammo obtained at-base, since you're diminishing the in-field stock, which will need replacing, expending logistical support. I'm thinking maybe 25-50% more.
Edited by William Petersen, 04 April 2012 - 10:02 PM.