For screenprinting, I'd say a number is a number.
For heat seal, at least in my shop, two digits is charged more, but usually not double as though it is more material, perhaps more weeding if cad-cut, but same application labor.
Now, what are relief numbers? Are those the guys that get reversed out of an oval or rounded rectangle?
If you are using paper stencils, I'd still say same price, and if vinyl letters as the reverse stencil, roll the extra nickel into the total.