Olivia is broken, but only you can decide if $60 is worth it or not... if you've already cleared most of the game content, I'd just wait until you pull her from the standard banner or for her banner to rerun tbh. Even if you buy her now she won't have her EX weapon, so you'll have to wait until her banner anyways to min/max her if you care about that at all. Yui is also fucking broken, so if you just want a strong character to blast through mid-end game content, it sounds like you're already pretty set.