That's what I wrote.
The gentlest washing cycle I know of is to throw it in the bathtub with warm water and your favourite degerent. Stir occasionally with a broom handle. Drain and rinse, etc.
We have a King size zip-on bed topper for our water bed and yes it's about 400 CHF to professionally clean every couple of years. The thing weighs a ton and would not fit into a normal or even commercial washing machine, that I know of. This may not be exactly what the OP was referring to. However, if you have a bed topper of any kind, one could remove it and use simple baking soda to absorb stains, orders, etc. Sprinkle it on and let it sit for several hours. Standard-sized boxes of Arm and Hammer Baking Soda are available in Germany for about 1 euro per box. Possibly spraying it with a coat of vinegar and resting in the sunlight and wind could help as well.
Just simple advice on how to clean many things. Vinegar, baking soda, sunlight, the breeze of the wind to dry. Check Google for more refined recipes.
I have done that on occasions- using feet too.
But please, how do you drain sufficiently, and dry, a thick, heavy, bed topper 2 metres by 2 metres in an apartment?
No wonder some have mould problems!