Sale 1: Dec 6

https://imgur.com/a/L4w1SRn

sold for 33 keys

Sale 2: Nov 7

https://marketplace.tf/items/tf2/228;15;u702;w4;pk412

90/1.84 = 48.9 rounded to 50 keys

However since other Isotope Secretly Serviced skins are starting at 37 keys for MW and FT so it is safe to say that a well worn would have to be less than that.

In this case I'm going to use the average from the lowest currently listing, and sale 1 which = 35 keys