There's six in existence, and G. Energy has a seller at 35, but G. Energy is also a lower effect in rank. Low-tier effects are placed decently high on this cosmetic, like Miami Nights, for instance, with a price of 35. This price should be larger than other recently priced effects for this hat, as P. Energy is in the top 25 for unusual effects. Ultimately this unusual should have a price at around 41-43 keys, and, if I was able to, I would price it at a lot higher. Although there is one seller at 40, Haunted Phantasm Jr. has a price of 59, and Phantasm is placed at a way lower rank than Purple Energy.

https://backpack.tf/unusual/Fat%20Man's%20Field%20Cap - Unusual Prices for the Fat Man's Field Cap