Mine isn't quite needing replacement yet (just some cracks in the outer layer; the inner layer and visibility are fine), but here's what I found in my area when I looked...
Speeyglass.com: $200.13 + tax, installed
Safelite.com: $230.95 + tax, installed
Others range from around $250-$325 + tax, installed
As for allowing Lexan, that could be nice because they don't crack. However, they do have their own durability issues, so we would need to use tear-offs or something to make them last, and they do seem to cost more money...
LindseyRacing.com: $421 + shipping
ApexPerformance.com: $425 + shipping
HrpWorld.com: $413 + shipping
At twice the price of glass, and with an obvious weight balance advantage, I can't say that really fits our "low cost equal racing", even though I'd probably get one myself if it were legal. But I suppose there could be a strong case for allowing them if someone could find them for the same price, or less, than glass...