The first time they did this, (2 months ago?- the one where the girls were stage 10) was the first time I did the championships since they appeared

As a F2P player, I had roughly 1000 kobans, 200 champion tickets, and roughly 1000 items in inventory I never bothered selling

I flat gave up after I hit the champions stage. (40 matches that time also) The koban to avoid "resting" I could avoid, by simply waiting, but then I optimizied my group/battle stats (I don't PVP either), and trashed the stage the next round, and hit that 24 hr brick wall. with only 1 fight availible, there was now way I could get more than 10 fights in 7 days, much less 40.

So this time I had about the same stats (more kobans, fewer champ tickets). I noticed the girls came earlier, and they eliminated the "give girls experience" levels (Probably too many complaints about girls already being overmaxed, so you couldn't give them exp even if you leveled a couple times), but left in the championships.

No, in my opinion the prizes aren't worth the kobans to finish them
For those reading this AFTER the event finished ( may have some tasks mixed up- after you beat them, they don't show up)
Total length of event 7 days- double prizes for 1200 kobans

level 1-prize 20 energy (Finish all daily quests if I remember correctly)
level 2-prize 5 fights (win 15 times pvp/tower- again, easy task- beat up the 5 weakest players in tower, doesn't even cost fights/energy)
level 3-prize 10 spins on cash pachinko (sell 20 items)
level 4-prize 10 fight tickets (beat 30 villians)
level 5- prize 10 champion tickets (spend 1.2 million (bought out affection store), I again finished tasks 1-5 easily within 3 hours (Daily tasks/waiting for fight energy were most annoying)
Level 6- prize 2 million, 40 champion fights (the prizes from winning champ fights still meagre-cash, junk items to sell, pachinko orbs (that you get from daily tasks with less effort), vanity items, and 1 girl (if you're lucky- and only 5th round). Losing the fights give you the 15 min rest, and a little cash, so 1-10 hours if you lowball it, depending on how many challenges you have opened- for me, it will take 3 days (2 challenges opened, 5-10 fights to finish a stage)
level 7- prize not sure, gold fog--mythic pachinko orb, worth 25 kobans? or maybe the 3 round one, worth 140 kobans?- sell 50 items
Level 8- prize- first girl, 40 tower/pvp fights (about 1-2 days, without kobans, if you don't fight villains)
Level 9- prize 20 fight pts- spend 4 million
level 10- prize ?? purple fog ??- fight 100 villains (3-4 days, depending on if you saved your fight energy during step 8)
level 11- prize-20 champ tickets- 60 PVP wins (2-3 days)
level 12- prize-5 million, 80 champ fights- depends on how many tickets you have (my estimate is 7-10 per day, depending on buying them with energy, and getting lucky in village fights)- but roughly twice as long as stage 8
level 13- 5 mythic pachinko orbs (450 kobans value)- spend 8 million
level 14- 2nd girl- 70 pvp fights

My analysis- event will end while I am on stage 10, and I'll never find out what the purple fog is (3 days stage 6, 2 days stage 8, run out of time doing stage 10 (2 days remaining to do task of 4 days). Even lowballers who don't finish all available champ stages within 60 fights (and thus avoid the 24 hour penalties) will probably still be doing stage 14, even if they pay the 1200 for double prizes (and thus extra fight energy)

If you don't mind paying for pachinko pulls, instead of waiting for the freebies from daily quests/villains
If you can get to stage 13, the double prizes are probably worth it (extra items worth 700 or so kobans, + extra girl(900 kobans))
If you can FINISH stage 14, double prizes is even more valuable (700 or so kobans, and 2 extra girls (1800 kobans)