I would say seeing as my fairly old taunt the baloonibouncer is currently priced 37.5 with a megastrike a newer taunt which is somewhat more desirable right now for pyro would do well being priced at 40. i wouldnt sell it for any less than this. the steam market shows it being sold for around 35 keys but as the unusualifier is still more expensive than the actual taunt i'd take that with a pinch of salt.

https://imgur.com/a/UKL9f - as you can see there are two listed and only one is more expensive than this unusualifier

http://steamcommunity.com/market/listings/440/Unusual%20Taunt%3A%20The%20Skating%20Scorcher%20Unusualifier

https://imgur.com/a/Y5sv7 - bp.tf classified

i believe that due to the price of the unusualifier and the effect, that it should be a higher price (right now) than the lowest price of the unusualifier there is also a backpack.tf classified also listing this taunt for 40 keys thus my suggestion of the 40 keys initially price suggestion.