You could build a floating diaphragm valve for that price or less, I'm pretty sure. A square YARD of 1/8" 70 durometer commercial grade neoprene on mcmaster is around 10 bucks, and I think that's enough for something like 81 4" discs. You could get a smaller piece of it for less. A square foot would be plenty, I think its something like 3 bucks for that. Then get a square foot of 1/32", that's more like 2 bucks. Then a small section of 16 gauge sheet metal... totals to around 10 bucks probably. BUT, it involves a lot more time and effort to making your first homemade valve.. believe me, been there, done that.
Actually, after going through typing all that I'd go with the watermaster, at least to start out. Mod it to be pneumatically actuated. Pushbutton or blowgun, either way.
edit: Also, what is the cfm on the "air compressor"? They vary greatly, from the 12v emergency compressors you plug into a car, to small electrical oil-recycling ones like I have(around 1.3 cfm), to compressors with tanks. They also have some that you put gas in and they work anywhere, but I doubt you have one... its something crazy like a thousand bucks.