Was in Walmart the other day and came across this Ecobee3 thermostat in clearance for $100. It's pretty much the lowest price posted here by atleast $50 apart from getting it via an electricity company.
It includes 1 sensor to keep on the second floor.
Posting a picture of the item, not exactly sure of the SKU or UPC, maybe someone can figure it out based on the clearance sticker.
Ofcourse this is YMMV so dont go crazy on me.
Product link: https://www.walmart.com/ip/Ecobee.../176109857