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Well, Heloise is an extremely powerful card. She will generate a lot of wins, and might be necessary to get into the Top 100. She might make back the investment in a healthy amount of time, actually, if Leaderboard prices are calculated. But that's just me speculating from Diamond 1 :-D

Maybe, but not alone. Let's say I buy Heloise. I don't have most of the Rebellion set. I have none of the Hall of Legends promos and I have only a few of the commons, rares and epics maxed out. I do have several legendaries maxed out: Daarg, Rage, Lorkus, Tofu, Venka, Nalara, and Akane. And I have the 2 pre sale promos: Mantaroth and Grimbardun Smith. If I spend all that money on one card but I don't spend a ton more building my Rebellion collection and also on the next set, I still won't be able to compete for top100.