These mass produced plastic sleeves that need cost a few pence, if that, are everywhere and the minimum price always seems to be £10. There are shops that sell nothing else. The mark up must be almost the entire price.
Look at the upper end of this market where John Lewis are selling one they say is ‘lifeproof’ for £60. A waterproof one is £100.
While people with unthinkingly overpay without a care in the world, why need manufacturers and retailers even think about offering genuine value?