Nah, it's just a demand thing, if they would use other materials more, those prices will just go up.
When Maguuma Lily's were "hot" in HoT, they were sold for way higher prices as well.
Prices will drop again (I just refuse to make mushrooms, but I refused to for my GH before as well)
More decorations are coming with the updates/events, maybe there will be cheaper ones to make, but likely prices of materials will go up again 🙂