Not gonna argue with you, but I can tell you that I got 50 Rare Souls from a rank 6 offering. So it's not that bad even you hit those. However, I have no idea if that is always the quantaty handed out. The "rewards" discription is not really helpful in that regard, since you get to know neither chances nor quantaties. No idea how we should figure out if we should prefer 1 bigger or 2 smaller sacrifices.
I don't know if it is a bug or left out, but in the screen shot posted in the announcement thread the sum of your sacrifices and your vortex level are displayed. This is not the case for me.
It would also be great to display the sacrifice value of your characters in the team selection where you upgrade them. At the moment you have to switch a lot back and forth to find out you are still very, very close to reaching the next tier.