For Strange Unusual Burstchesters (this is the only Strange Taunt), the Strange variant can be priced if the Strange Taunt alone is worth considerably more than the Unique Taunt. Additionally, the Strange Unusual variant has to consistently sell for more than the base Unusual.

For the vast majority of Strangified Unusual sales, those sales are approximately the same price as non-Strange sales. Some of the Unusuals that are Strangified (primarily the Veil and Bonk Boy) are expensive as is, so the Strangifier's additional value is negligible for suggestions in those cases.

Some examples of cheaper Strangified Unusuals selling for roughly the same price as the non-Strange:

https://backpack.tf/suggestion/5f525f430150e970151dbc7f

https://backpack.tf/suggestion/5d5ab7086780727d093784a7

https://backpack.tf/suggestion/5b7fafcb44325a74a002cceb

https://backpack.tf/stats/Unusual/Sole%20Mate/Tradable/Craftable/33 (granted, the non-Strange one has never sold, but still)

https://backpack.tf/stats/Unusual/Sole%20Mate/Tradable/Craftable/8

https://backpack.tf/suggestion/5873ff06e338770fe8165378 (23 was the non-Strange price at the time)

https://backpack.tf/stats/Unusual/Deep%20Cover%20Operator/Tradable/Craftable/6

https://backpack.tf/suggestion/5ea71ed56b35de56ac03e217

A case could be made to price Strangified Unusuals separately if the Strangified variants sell for consistently more than the base Unusual, but the "price all Strangified Unusuals as the base value" is a blanket rule that covers the majority of cases. Burstchesters don't fall under this blanket because the Unusualifier is applied to a Strange Burstchester that, at least currently, sells for far more than the Unique Burstchester. Strangifiers are applied to Unusuals that aren't comparable to the Unique item.