Thanks.

Yeah we're on the edge of zone 5/6, so fairly cold in the winter, we do more heating than cooling, and a high r value is desirable.

I did some research after posting my question, and it seems like a cheap way to get sealed and high r value is to do what you said and cut poly to fit, and then seal the edges with spray foam. But then instead of doing more layers of that, just put a few inches of unfaced batting on top to boost the r value. I'll probably go that route, do the whole basement for $50-75 instead of $500.