How to use existing property in expression?

[AlphaCentauriBear]

Hello, fellow developers.

I want to proportionally change cargo prices. I don't want to set them to certain calculated value just multiply them all by a factor.
Let's say for starter I want to double cost of coal. How do I do it?
I understand how to set property to constant but how do I retrieve existing value of the property to multiply it by two?

Another example would be to set train running cost based on their speed/power or something like that.