People have pointed to XL engines having high maintenance costs, but the majority of your repair bill will come down to two of the four check boxes on your list: Armor and Ammo.
AMMO:
Depending upon what loadout you prefer (LRM users have seen this, especially with Artemis. SRMs are insanely expensive to rearm as well) HALF of your R&R will be rearming. Imagine the outcry if you had to purchase your ammo every single time you came out of a battle, instead of the 25% of your ammo.
People have already started getting around this problem, though. They've dropped a heatsink for another ton of ammo, then just taking the free 75%. THIS SHOULD NOT HAPPEN. There should be no reason in which hurting your team (volunteering to lower your maximum possible damage) for personal profit should ever happen. But this is only the tip of the iceberg, for even if you don't rearm, you still have 75% of your ammo, and you still can be effective in your team. But if you don't repair...
ARMOR:
Armor is unexpectedly and unreasonably expensive. One ton of armor (32 points for standard) costs 10k. Just one ton. That's only a third of what the Atlas uses for its front-center torso for stock mechs. If nothing else was damaged in the fight, a cored Atlas would pay 30k just for armor, just for its center torso. Of course, you're in an Atlas. You're the biggest target on the battlefield. Personally, I don't run any Atlases, but if an Atlas were to take proportional damage to my Cent or my Hunch, it would be missing arms (2 tons of armor each), a side torso (2 tons), and other various damage (2 more tons of armor destroyed between head, legs, and back). Total tonnage lost? 11 tons of armor. 110,176 CBills. And that's with standard armor. If the Atlas were to take Ferro (what? Why? Good thing Ferro is terrible compared to the savings of Endo...) that bill would be 219,989 for the armor alone.
Good thing we don't have to pay for 75% of that, but even with 75% free repairs you still have 30k of armor repairs (or 55k for Ferro).
Compared to the Maximum amount of armor on a Jenner (238 points, 7.4 tons), you're only spending half of what the Atlas spends on armor IF EVERY SINGLE POINT OF ARMOR WAS STRIPPED OFF BEFORE YOU DIED.
If you want to know the real reason you see unrepaired mechs on the field, look at the armor cost. If there is a system that rewards people for being incompetent (read: What Communism actually does), people will be incompetent and be rewarded.
If I were to design a system, I would first bring armor and ammo costs down across the board, then I would reward players per match as a function of how armed/repaired they were when they entered the match. If you were 100% rearmed/repaired you get 100% of your earnings. If you were 75% rearmed, you would get 75% of your earnings. If you were 75% repaired, you would only get 50% of your earnings.
Rewarding players for mediocrity will only result in mediocrity.
*Edit: Grammar
*Note: I'm not saying that XLs aren't expensive to field (I just landed a 46k repair bill on my engine and I *didn't* die), just that Armor and Ammo is what really hurts the team when you don't repair.
Edited by ArmyOfWon, 13 November 2012 - 10:12 AM.