Check out primsasonic and panamorph for pricing, or call Jason at AVS. I'm not sure if AVS deals in these or if there are any power buys going on right now. Lenses are not cheap. But, if you buy a good one, you should never have to upgrade it. I'm still kicking myself for not getting in on the prismasonic power buy before Christmas.
Alternatively, you can build your own very inexpensively. Search DIYaudio.com and you will find the thread that will teach you how. You won't get non-reflective glass, so you may get some reflections on your screen from the lens, but you can make a pretty decent lens very inexpensively.
I don't know about the Benq's beam size. Best to ask the lens manufacturers to be certain.