I know this is just anecdotal evidence, but my mom bought me some Target brand Xhilaration pajama pants in 2005, and I still wear them. I've never had to mend any holes. The waist is a tie rather than elastic, so that hasn't worn out either.

They don't even make that brand anymore, but if you see them in thrift stores, grab them.

I've actually got my eye on some Ralph Lauren satin pajamas if you're looking for something with more guaranteed quality.