Edmund optics has a 50.8 mm sapphire window, part number NT45-817 As does CVI Melles Griot PW-2012-SPRWSA-5002M-SPR
none are the prices are very good, to say the least, but those are very, very, very flat (the CVI ones are 1/10 of a wavelength, so flat within 60-70 nm ) Hence the OMGWTFBBQ price. It'd be nice to find something of lower quality, so the price was a bit easier eh?
Now if youre talking mini-mags, the price is still steep, but not so insanely high.