We didn't give a cash bonus (honestly, she lost her mojo around December and it was all I could do to get through to the end) but we did offer to send a box to Germany with some of her stuff in it and that ended up costing nearly 200 dollars - so pretty expensive gift in the end. Got the idea from AuPairMom
Also
Have her clear off the cell of her personal stuff (pics, texts, contacts except you and DH)
also reset the vm password to 0000
Get back library card, Y membership card, etc
make sure she returns any books/media to library, videos to red box