Sale at $9.99 Marketplace.tf on April 25 2020:

Median key price on April 25: $1.79, 9.99/1.79=5.58 keys

https://marketplace.tf/items/tf2/1112;5;u3032

(Other sale at $9.99 on April 9 but can't be used because it's older than 3 months)

One sold for Earbuds + 4 ref

https://backpack.tf/profiles/76561198302161698#!/compare/1589414400/1589587200 (May 14-May 6 2020)

https://backpack.tf/item/8371488813

Earbuds value: 2.75 keys (2 keys 43.55 ref), adding the 4 ref makes 2 keys 47.55 ref, 2.82 keys

One sold with a Schadenfreude (1.7-1.75 keys) for 4 keys pure (removing the taunt, 4-1.7=2.3, 4-1.75=2.25, leaving us with a range at 2.25-2.3 keys

https://backpack.tf/profiles/76561198335837512#!/compare/1589068800/1589414400

However, there are buyers on marketplace;tf at $6.60 (taking key price $1.93; 6.6/1.93=3.41 keys), the sales at 2.25-2.3 keys and 2.82 keys would be too low if people are already offering more.There are currently no sales to confirm 3.41 keys so it can't be included in the range