Asking prices will tend to be lower during an event, because people are hungrier for Elixirs/Potions so as to chase point or rank rewards. So hang onto your consumables until partway through Soul Hunters and watch the Rare Card report boards for people advertising deals.
Note that I believe the next event is driven by Potions, so be careful on the exchange rate. There are times that people will trade potions and elixirs 1:1 and times when one or the other is valued higher; use these fluctuations wisely.
Fiona is the cheapest but also the bottom of the 5* barrel. The Dream Breaker 5*s (Rem, Sogno, Hrwm) go very low; get either of the first two before Hrwm.
Meg was the poster child for "cheap but good 5*" -- a few limited editions in a row were heavily overbought and even their quality 5*s became devalued. You can probably still snag a very cheap Meg if you're persistent. If you can tack on a maxed 4* or some more consumables, Kiki, Yuki and Mihiro are in a similar boat as undervalued but solid 5*s going under 100TE. Kyo as well, probably cheaper yet, but Rem will cost less than Kyo and is arguably superior.
Among the 'basic set' and early edition 5*s, those that are in low demand have dropped substantially from their original asking prices. Marguerite, Lucrezia, Adolph, Thrain, Gerhard, Emma and Elena are cards that either weren't great to begin with or that have been dwarfed by later releases. To a lesser extent, Serafiina and Camille also dip below 100TEs regularly.