This is why the "staying over" rule was invented. It goes like this:
Ex - "Hi can I stay at your place for a few days? I am out of a place right now.."
You - "Oh yea? Sure you can come over.."
This is where "you" fucked up. After the "Oh yea?" you should have asked...
You - "Oh yea? Are we gonna fuck?" <-- the 'payment' for your place
See.. you my friend have something that she wants, mainly your place. Since you obviously don't have a serious relationship going where another peice of ass is currently staying at your pad.. you need to ask the question directly. Its quite pointless for someone else to come into your place unless they are willing to exchange something for "your goods". Since this is your ex.. its her ass, because we both know money isn't really a good way to barter for your place.
Oh yea.. and make sure you mention that its not going to be a "wam bam thank you mam" kind of fuck either.. its gotta be a couple of good fuckings! That way she knows shes paid up AND you know your time/place wasn't wasted.
Good times!