To be honest, I don't know. I suppose it costs so much because a throne is very rare. The chances of you going to your corner shop and seeing a throne for sale are very rare !!
And it is a large item.
So yes I agree maybe he could do a bit more than sit on a throne - but there you have it what other reason could you use a throne for? I don't know!!
I agree it is very rare... But luckly I got a throne out of a chest for free. I dont know if this is cray or not, but I gave it to my friends tama as a present.