Whew, pretty straightforward suggestion for this one, evidence follows:

MPTF sale: https://marketplace.tf/items/30893;5;u10/Purple%20Energy%20Classy%20Capper

Sold for $149, which is near exactly 75 when pricing keys through MPTF, or 80 when pricing through BPTF

BPTF Classifieds sale: https://imgur.com/a/vYDt2 - Sold for a straight 75 pure (to me)

3rd sale: https://backpack.tf/profiles/76561198042003888#!/compare/1521590400/1521763200

Sold for a Tesla Coil BoA (~78), with 3 keys added to the Capper - brings it to 78

https://backpack.tf/profiles/76561198119013032#!/compare/1522800000/1522886400

2:2 with Starstorm Slumber Warmer and Vivid Plasma Sultan (Both outdated though, even some Warmer sales are discrepant - one of which being unclear)

https://backpack.tf/profiles/76561198086472000#!/compare/1523059200/1523491200

56 pure sale - sold within a week with no visible buyout - marking this one as an outlier when compared to the other sales at 75

Hopefully that clears everything up, all credit to Foamy for finding the bulk of these sales!