More of an FYI than a price.
I had to add quite a bit to a Cronus to get my hands on one and took about a year of actively looking. All the others always had a few interested buyers so you can definately get more than a direct swap for Cronus which is a common offer.
I think timing is key here too. I was personally able to get away with a 1:1 swap Cronus for Baal by approaching someone who was about to earn their own Cronus for the first time. They were going to make the combo anyway, so it was a good trade for both parties.
That said, I have no idea what to price a Baal or Cronus at on the best of days. Baal is certainly rarer, but it's not unobtainable given new battlers will earn their own eventually.