Tiamat has already been proclaimed the best commander card in the Forgotten Realms set, and rightfully so. It has a powerful ability that will make dragon tribal players really happy.
The prices have been going up and down for Tiamat in the last couple of weeks. It started at $20, went up to $40, and corrected itself at $25. During the release week it may spike to $40 once again.
The big deal about Tiamat is that it also influences the prices of other dragon creatures in MtG. So there is a lot of room for that price growth in the coming months.