Updating my first suggestion with more information provided by Rainy.

For the high value I considered the sales stat from https://marketplace.tf/items/tf2/31262;5;u224 since march 2023, for key prices in USD I'm using the median price found at https://marketplace.tf/items/tf2/5021;6 of the same day the hat was sold.

1. 03/02/2023: 1 sold at martketplace.tf for 31.99 USD, key was 2.05 USD ~ 15.6 keys

2. 03/15/2023: 1 sold at martketplace.tf for 31.99 USD, key was 2.03 USD ~ 15.76 keys

3. 03/24/2023: 1 sold at martketplace.tf for 31.99 USD, key was 2.1 USD ~ 15.23 keys

4. 04/07/2023: 1 sold at martketplace.tf for 31.99 USD, key was 2.1 USD ~ 15.23 keys

5. 04/11/2023: 1 sold at martketplace.tf for 31.99 USD, key was 2.08 USD ~ 15.38 keys

Decided to round up the high value to 16 keys, considering the scenario you would take a key as 2 USD. Also the most expensive sell listing of this hat currently is for that amount: https://backpack.tf/classifieds?item=Undercover%20Brolly&quality=5&tradable=1&craftable=1&australium=-1&particle=224&killstreak_tier=0

For the low value Rainy shared the sales statistics from Gladiator https://gladiator.tf/sales?item=Crustacean%20Sensation%20Undercover%20Brolly&at=2023-04-14T05:20:47.913Z where 4 were sold for 11 keys in the past 30 days. Screenshot: https://imgur.com/a/8NiLhrW

04/13/2023 I bought one for 13 keys from a listing from this site. Screenshots: https://imgur.com/a/S2xgWWh, https://imgur.com/a/5Jl1Lwd

PS. English is not my first language, sorry for my grammar.