There are some incredibly expensive cards one can potentially find online, but a lot of them are very old cards, maybe playtest versions of existing cards, or mint copies of Black Lotus
With a host of powerful new mechanics like the new team cards that feature two different characters in one card while allowing them to Shift onto multiple targets and gorgeous artworks, Attack of the Vine
The fact that her ability targets helps out with all the Valiant effects in Boros Mice, too
it has some very good cards at common/uncommon (Choco-Comet, Thunder Magic, Suplex, Call the Mountain Chocobo, Samurai's Katana), but more bad filler than any other color
Then, when you ping the opponent, as long as Ballista has enough counters to survive, youll gain a life, triggering The Destined White Mage to put another +1/+1 counter on Ballista
Any cheaper than that and this starts to become absolutely ridiculous