Trade Lumia 800 against the Galaxy Nexus and tips?

Hi there,

I want to trade my lumia 800 against the Gnex because I wanted to try something new. But how much money I have to pay extra with the trade? I payed 275 euros then for the Lumia 800 few months ago.

And another question: Should I stay stock Jellybean or install CM10 or another rom?

Greets from the Netherlands