I am in your area and while I won't discuss pricing out here in the main forum, I will say that you are not giving us enough details.
What is the substrate? Aluminum? Alumalite? Wood? Or is it RTA vinyl?
I will tell you that good old Fellers sells reflective by the yard. If using Glantz or Pioneer you will indeed need a full roll.
But if you've not used reflective (I only use Scotchlite) you may want to buy extra.
Some plotters require a different blade for cutting reflective.
You want to allow for that too, and make sure to do some test cuts.
Refective can be a PITA to weed, too.
And I don't price by the square foot, maybe sometimes for banners. But $5 a square foot for
signs? Yikes. I got more than that back in 1990.