They really don't drop much, but salvage can still be useful. It's the massive engagements that really pay off. Missiles are the most common drop, and some of them fetch a nice price. Cleaning up after a battle between factions can give you a nice monetary boost early in the game.
Once in a rare while you might find a nice expensive weapon too.
Drops are rare since I believe that only things in the cargo hold can be dropped. So to get a weapon, the enemy ship has to have it in its cargo bay, and not equipped. This is why missiles are by far the most common drop, since the cargo bay is the only place they can be.
No and no... also, o god, no.
You're not meant to blow stuff up hoping for "ph4t l00t".
How to make cash:
Remove shields, tickle the hull down. A freighter is very likely to give up his freight in hopes of buying free of your attention.
Pick up the cargo with a freighter of your own; whatever fighting craft you use will most like not have enough space.
Continuous tickling may lead to the pilot bailing out in an attempt to save his life by giving up his ship.
Fly close to the ship, exit your own (shift+e, confirm), fly within 30m, target the abandoned ship, press i and "claim".
Abandoned ships may be anywhere from empty to combat ready.
Large craft may require marine induced takeover.
Mods can make capping easier, harder, more reliable, and there are modules to allow claiming from within the safety of your own ship and at larger range.
Now, if you destroy ships, you generally lose most if not everything aboard to the explosion.
A couple missiles are left behind commonly, but the big stuff usually gets blown to bits.
However, you may be lucky and have them leave behind full sets of turrets and similar goodies.
Not as good as capturing the ship, and very infrequent, but depending on the weapons aboard this can be worth millions as well.