I did calculate this, but you're probably recalling my results for Advent events. The BP recovery per ticket is about 0.14, with 5 tickets earned per Expert MVP (joined with BP). That's 0.7 BP earned, with a cost to join of 2 BP. However, that doesn't take into account event rewards, nor tickets earned through AP usage. Conversely, the sustainable way to do raid events without a huge devotion of time is to join, auto, and do something else. You're not going to MVP every time with that strategy, no matter how good your team is. But certainly you can get sub-MVP most of the time.