I think if normal high-end smartphones are in the $1,100 range that these foldable one swill be $1,500+. Honestly, I think getting so many people to buy $900 phones was only going to work once or twice. I think the sub-$500 phone will come roaring back in 2019. I think Apple, LG and Samsung will "want" to offer phones with foldable screen,s but they won't be able to do it anytime soon for a price even 2% of the market-will-bear.
I own a S8+ and I can honestly say, even thought I like it, I won't buy a $900 phone again.