If all that exists is matter, then what kind of property would 'value' be?
If all that exists is matter, then what kind of property would any property be? I have trouble seeing how any property could itself be a material thing. A red apple is a material thing, but is its property of being red itself a material thing? Fortunately for me, not even the physicists say that literally everything is matter: there are also fields of force, states of matter, quantum vacuum states, etc. In any case, the claim that everything is matter seems to threaten not just the property of being valuable but every other property as well.
If happiness, for example, is simply a mental state, no different from sadness or pain, then how can it have the property of 'value'...
If happiness, sadness, and pain are all mental states, then they don't differ from each other in respect of being mental states. But that still allows them to differ in terms of whether they have the property of (positive) value. Not all mental states must have all the same properties, just as not all material objects must have all the same properties. In sum, "no different from in one respect" doesn't imply "no different from in any respect."
I'd also caution against the use of reductive language such as "simply" (or "only," "merely," "just," "nothing more than," etc.). It can sometimes obscure important differences. See my second reply to Question 5626.