The issue being the ridiculous price variability and sloppy configurations of X299 motherboards. Prices range anywhere from $258-$489. At the low range :
https://www.newegg.com/Product/Product.aspx?Item=N82E16813144058
If Threadripper boards come in under $258 and have a upper lower ceiling, that is what would be of note.
I do imagine a $200 or even lower threadripper motherboard (double the lower avg. price of a Ryzen 8) board.
Costs add up and AMD is doing well in K.I.S.S.
Skylake is all over the place.
So far, the motherboards that have been previewed for TR have been pretty high end. As with x299 motherboards, I expect that there are a whole range of feature sets, typically offered at different price points. So, Skylake isn't all over the place - it's just a standard range of offerings. AMD's motherboard partners will not go with KISS - they are in it to make money.