I've had the questionable honour to find and use the L85, P90 and FAMAS.
But why are these weapons so expensive?
If anything, I found them to be worse than the M16 you start with.
Am I missing something here?

Also, the L85 was removed from my armoury again, 'running out of stock'.
I feel that you should really get to keep the stuff you find. It's hard enough to get a briefcase as it is. (Or maybe I am unlucky.)

I'm not sure if JackMayol has been tweaking the weapon parameters lately, but the last time I had P90 I had a huge streak. It's one of my personal favorites apart from AKS74U. They change the game completely for me.

M16 on the other hand doesn't feel that good to me in the first place even, I'm always even switching to G36 when I come across one.

L85 and FAMAS seem to not be so well configured as reward items, as it seem I'm almost never going for those weapons. SG552 is pretty cool with its single shot mechanism, felt like a killer when I had it. L85/FAMAS seem about on-par with the stock weapons, maybe a bit better, but nowhere near rare grade, so I'm most of the saving myself from the hassle of obtaining one.

We recently discussed about an option to make the reward weapons L85/FAMAS/SG552 even better, rare grade, but make them also even more expensive.

In all honesty, I didn't use the P90 on enemies. I just emptied the clip and saw the spread it had.
So I thought to myself: Meh, I am not going to hit anything with that.

It would be nice to see these guns be really special. Special enough that they really give you an edge over regular stuff and really make you curse and cry if you lose them. I guess I'll just wait what you'll be doing with them.
Thanks!

Personally I'm pretty much like that, I'll do everything I can to get back to the weapon if I get killed and drop it, as like I said, to me it feels they seriously change the gameplay completely. There was one campaign playthru for me when I found a Vintorez and I didn't dare to use it as I was afraid I'll lose it. Finally I took it into use in the final map of the campaign as I felt I was going nowhere there, and I ended up doing ~140 kill streak doing the backdoor missions, eventually gaining us enough area for more troops and turn the tide of the fight. Felt awesome and I was near death so many times, only saved by vests & paratrooper calls with medics.

I didn't make a change to those weapon mentioned in a while apart from the P90 which I nerfed a bit, but that's still also a long time ago. The P90 is definitely better than a stock weapon, even though it looses it's effectiveness at longer range, which is fine.