Does anyone know what determines the amount of HP you get if you should sell a 4*? E.g. my Kingpin at 4/3/4 lvl. 180 sells for 0 HP, whilst my Wasp 5/2/2 lvl. 120 sells for 1500 HP and Gwen 4/1/3 lvl 172 sells for 3000 HP. Not that I want to sell, just curious. 
If they have recently been changed, the buyback price is increased. Since Wasp and Spider-Gwen just got updated they sell for more than the other fourstars.
Gwen and Wasp are selling for HP right now because they’re just had their powers updated. This is so that people who don’t like the changes and are annoyed at having sunk resources into them can claw something back.
Ordinarily, characters don’t sell for HP until they have champion levels.