Agreed. The first problem is that you don't want a vapor barrier over bats in a basement as that will trap moisture coming off the concrete. Using xps as the vb
that way may make it worse if it moves the condensation point into the fiberglass.
Do you have any space behind the studs?
Have you looked into the DIY spray foam kits? Like foamitgreen.com? Moe expensive than xps, but it would seal tight to the studs and even if you only got an inch in there, you could use bats on top.
I've never used it and I haven't seen much in the ways of review here, but from what I understand of spray foam, this might work and not be too much more expensive than xps sheets with tape, caulk, etc.