P = (# of rentals * $1) - $32
where $32 = the $20 disc cost + monthly var cost
If redbox rents the movie once per week, their per disc annual profit is $20
But, if redbox can now sell the virtual download code for $8 online (essentially $0 variable cost for holding digital inventory), and digital sales has little impact on physical rentals, then the equation would be:
P = (# of rentals * $1) - $32 + $8
With annual profit per disc in the 1x/wk rental case now $28, or a 40% gross margin increase. All #s are just napkin math/assumptions, but it's easy to see the benefit here for redbox.