This is a really good deal for an upgrade. There are some pretty serious issues with using older version of Bento on Lion and this is a great way to get past that. I'm not a fan of their typical upgrade pricing since I don't think the iterations have provided much new value.
Unfortunately for me, I already purchased Bento 4 on the MAS because of the bugs with Lion. I have a couple of Bento databases that I use a lot and needed to make sure I would continue to be supported on Lion. I think this discount is a way for them to avoid patching older versions and just get everyone on one code base.
One gotcha though. If you want to upgrade a family pack, you need to purchase 5 upgrades at $9 each. That's not an oversight either. It's spelled out on the offer details. That's actually a pretty poor deal.