That's a really tough one. In all likelyhood if you are going to use a laptop for Octane rendering, it is going to be plugged in, not running on battery power.
There seem to be several 17 inch laptops with a 1080gtx that are at or beyond $3000.

From B&H in the U.S. those at or below $3000:

ASUS 17 ...

The HP Z series doesn't support SLI with the GTX cards (it does with Quadro cards) but you can use multiple GTX cards for Octane, 2 with one CPU, more require 2 CPUs.