Well explosives aren't really cheap as they have extremely limited ammo to account for the ability to multi-kill.
Think about the bullet to kill ratio here. The SAW for example takes (and I'll be nice here) 10 bullets to kill (Assuming your aim doesn't suck horribly) divide that into the 600 bullets that you carry by default. (Or was it 400.) Either way, that's 40 or 60 possible kills without even getting more ammo. That's not even possible with and RPG for example. 2 shots equal a maximum of 15 kills each as the game only holds 16 and you'd have to be on deathmatch with everyone clustered in one spot. So a nearly impossible maximum kill-without-restocking rate of 30 kills. And come on, who even get's more than 2 kills per rocket on average? Mostly it's just single kills.
Snipers aren't that cheap either. If all shots hit and are one-shot-kills the M1903 can have a maximum of 20, the SVD get's 30, the AS50 gets 15
, and the Dart Rifle gets 12 I think.
If the math were done for a minimum-bullet-to kill ratio divided into each respective gun's default ammo count. A lot of guns might be considered cheap instead of others. Even if (and sometimes
especially if) you factor in the
average bullet to kill ratio or ease of weapon use the results may surprise you.
Also, explosive travel a lot slower than bullets, making them possible to be dodged after they're fired.
Sure there's the mindset of "Oh just aim at the ground and get a free kill." But it really isn't that hard to aim a machine gun and hold the trigger down...
Explosives are cheap on MW2 where the maps are small with lots of choke points, and overlooking camping spots. Just add in perks like scavenger which drops an ammo refill pick up on each kill, and you've got a recipe for a Noob-Tube fest.
