Sometimes a Creature's Remains Are Valuable
It isn't often the case, and some DM adjudication's required, but dead creatures can be quite valuable.
Material and Focus Components
Some spells require creature parts to be expended (as material components) or manipulated (as focus components). Without these components, the spell can't be cast. Most creatures who cast spells will have these components in a spell component pouch, but someone must fill spell component pouches with creature parts, and those creature parts are likely among the spell component pouch's raw materials. So if the creature-killers can find a buyer for the parts, that's a possibility. The dead creature's value will depend on the campaign.
The spells animal aspect and beast shape, for example, require animal parts, with the latter making an especially persuasive case for selling at a high price a rare animal's corpse. It's also not just animals, of course; the spell infernal healing, for example, requires devil's blood.
Animated Dead Servants
This is where the real money is. Using the spell animate dead et. al. any appropriate corpse can be turned into a skeleton or zombie. Necromancers are busy dudes, so they often don't have time to collect high-end corpses for animating, so creature-killers can sometimes get top dollar for corpses, especially for monsters that would make particularly good skeletons and zombies. Again, the corpse's value will vary by campaign, but selling corpses to necromancers provides a reason to preserve the whole corpse rather than hacking it up.
Selling corpses to necromancers might also be considered an evil act by some DMs. And characters who do so absolutely will face such animated creature again later (or sooner!) in the campaign, but at least the protagonists will recognize the creatures. Smart adventurers might even trap such corpses to prevent the corpses from being used against them or make that part of the deal brokered with the necromancer upon the corpse's sale.
There's a whole feat for Dragoncrafting, and there are at least 2 dragon-based special materials, dragonhide and dragonskin. Again, their value will be somewhat campaign dependent unless the dragon's remains are treated solely as raw materials.
Some Creatures Are Themselves Specifically Treasure
A Small dream spider can be used to manufacture the drug shiver, for example, and even its webbing and eggs have value. A dead poisonous giant toad can be used to make the goblin booze bufo. One can "boil down and refine the corpse of a flumph" to "extract as much as 200 gp worth of rare components," and "Powdered gorgon horn is worth 250 gp as an alternate material component for magic items" involving strength and stone.
It might take some searching on the DM's part and some Knowedledge skill checks on the creature-killers' parts, but occasionally the dead creature will be the dead creature's treasure.