I have about $70 invested in my mod (material/labor). I purchased the material from a local plastics company and had them cut and make the radius corners for me. Home Depot, Lowes, possibly Menards, etc., also sell the material as well. I've heard that some Lowes stores will also cut the material for you. I had a friend say that some marine/boat shops also carry it.
As far as mass producing, someone beat me to it
The cost of a panel for the lower section of the door only, I would guess the material would cost around $15-$25 (depends on type/thickness of plastic). Tinted is a little more expensive than the clear stuff, and Lexan is more expensive than Plexiglas.
If you elect to put the panel(s) on the inside of the screen door "recessed" into the frame, you could also consider using "mirror clips" to hold the panel in place (the kind that hold our stick-home bathroom mirrors to the wall).
You might find a local hardware store where you can take your screen door in with the material and they can cut-to-fit.
The Lexan material is far easier to work with (less prone to cracking like Plexiglas), and easy to cut with a straight edge and fine tooth jig saw (even a radial saw).
Hope this helps.