Intel X99 mainboards (socket 2011-3) ranged from 200-600 us dollars. AMDs offerings typically go for less than Intels, but Threadripper has an gigantic 4094 pin LGA socket, so I guess this will negate the typical AMD price advantage regarding motherboards and we will see prices in the 200-600 us dollar range as well.

EDIT - and since Threadripper will have 20 more PCIe lanes compared to Intels i9 series, motherboard makers will hopefully take advantage of this to offer more IO, which probably also increases the price.