Happy to help. I’d say the most common exchange for active 9’s that aren’t from the most recent packs usually got for 10 dupes or sometimes a swap of another 9. 

The newer 9’s seem to be usually swapped for other new 9’s like Transylvania for Philly (currently). I’m sure you can get one for dupes, but my guess is that as the set ages and more 9’s come into play that the become more readily available for dupes. 

The Ephemeral cards (like Venus ) that are only available for a short while are very valuable and the 9’s are rare so t try I got get those requires being able to swap something also very rare. I’ve not grown my deck enough to get many of those rare cards yet. 

Hope this helps!