Their last two singles sold 86k-ish first week (172k-ish combined) and have since then sold about 110k? They will probably end up selling about 150k-160k before falling off the top 200.
http://wiki.jpopstop.com/wiki/No,_Thank_You!
If Ayu were to go against one of their opening or ending singles... I don't know if she'd be able to sell more. K-ON! is ending for good soon though, so there's not much to worry about in the future, haha.
***BUT Ayu is not going against OP or ED singles. She's going up against character singles that only hardcore fans buy. Even though that's the case, a lot of them still do buy the CDs. Usually these sorts of things only sell 5k or less... but with K-ON, it's like 30-50k, haha.