Hi Tim,
Nice -- like the idea of seeing how low the cost can be for a functional air collector.
If it turns out that it runs to hot for the poly, you might just go with single glazing -- with only one layer of the poly, the outside of the glazing is always going to have the cooler ambient air to help keep it cooler than the collector internals. Single layer glazing might get the cost all the way down to $38.50
I think you may need something to control the airflow inside the collector -- the pop can approach works and its cheap, but lots of work.
Window screen or perforated aluminum soffit material work well as flow through absorbers, but increase the cost. But, if you have well sized vents top and bottom they will thermosyphon nicely and don't need a blower -- so that keeps the cost down.
Just adding some baffles to make sure the air flows to all parts of the collector, and using the back wall of the collector as the absorber might work pretty well and would be very cheap.
Another variation. Build the collector right onto a south facing wall, and do away with the polyiso insulation -- just paint the siding a dark color. This won't work for vinyl siding (the heat would damage it), but should be OK for wood siding. My vertical collector built in somewhat this way only gets to 185F when stagnated in the summer.
The back insulation is not needed if the space behind the collector is a heated space anyway.
Another one in the spirit of cheap air collectors:
Have always liked the simplicity of this Mother Earth one:
THIS $30 SOLAR SETUP HEATS A 30 X 40 WORKSHOP FOR FIVE HOURS OR MORE EVERY SUNNY WINTER DAY
If you get the cost down to (say) $1 per sqft, and 1 sqft of collector harvests about 900 BTU per sunny day, the payback period for $2.20 propane would be about 1.5 months!
Gary