With the whole legendary Livitya aside and talking only about event girls, I personally have never had an issue getting any of them, they always seem to drop within the first 2-3 days of the event playing only of natural combat regen, Even now it's been 3-4 days since event started and I already have both Viola and Hildr so I always assumed the drop rate is pretty high for event girls even though the chances do not get better every fight with said event girls as opposed to normal boss girls. Just keep playing you will get them and Don't go using kobans till the very last hour, save them. As for the low payout on beginning battles, I wouldn't worry about the income, you are not in a hurry, you got lot's of time to build up your affection, just enjoy the events =)
Also I like to look at each event battle as rolling 100 sided dice, you got to get it to land on the 95 - 100 to get the girl and for boss normal girls it starts at needing 100 then every 5-10 battles it goes down one so you need 99 -100 then 98-100 etc etc.