I would've said "printer ink", but ok

fursuits

That ought to be a reasonably soft landing, provided you don't land on the head portion. That might have some kind of internal shell or padding or something to give it shape.

Steam gift cards

If steam ever gets shut down......do you think they'll release the games we bought

I'd say you keep all games you bought you just couldn't buy more

I would will it with dead children

mattresses

Liquid gold with a few pickaxes at the bottom

a synthetic fluid that is liquid at standard outdoor temperatures, contains oxygen (and thus is breathable if jumped into) and has the right density, viscosity, and other characteristics to be able to make an average human survive jumping into it from 10 meters
and considering said synthetic liquid would cost a bunch of money to make (for the equipment to produce a suitable amount of it) it also checks the "valuable" requirement, and after you're done with the experiment you keep it and you can simply sell it for a high per-liter price to get lots of money

...

Deoxyribonucleic Acid

The foam from twitchcon

why printer ink

oh right

it's overpriced

Some printer ink is like $75 an ounce. Imagine an entire pool