*plugs ears* Lalalalala! I can't hear you!
In my mind Stewie is NOT for sale. No, absolutely not. I can not, will not acknowledge such a thing! I can not have another horse. I can not have another horse. I have too many big huge monsters as it is.
On a serious note, I really can't give you an estimation on price. Around here, anyways, weanlings - even very, very nice ones - go for next to nothing. If you want to get anything out of them, you need to hang on to them until they are 2-3 and at least started. That's why I have never put Fiona up for sale yet, though I have never intended to keep her forever.