IF I were guessing, I would say next year (and I'm assuming you mean September), between $750-$900. Even if you could get the full price you paid AND if there is going to be an X-Plus, I'm guessing the plus will be around $1150-$1200 for a 64gb model.
Personally, I see a VERY small market for an X Plus except for the "phablet" crowd.
IMO, your poll is flawed. It should ask if a current X user would be selling to replace the unit with a PLUS model. I sell every year and have NEVER upgraded to a plus as I'm not in the phablet crowd. The ONLY reason I bought the X is that the screen size is almost as large as the plus, but very close in form factor to the regular size.
I bought the IP8 in Sept and then bought the X in late October, with the intent to sell it if I didn't like it. However, I liked it so I food chained my 8 to my wife.