Initially the chances of success in Special crates (formerly known as Platinum crates) were secret, but since sometime earlier this year we know that it’s a 1% chance along with a bonus of a guaranteed epic after 50 pulls.
This translates to a cumulative chance of
- 10.47% of unlocking from 11 pulls for 6,000 gems
- 19.84% of unlocking from 22 pulls for 12,000 gems
- 28.23% of unlocking from 33 pulls for 18,000 gems
- 35.74% of unlocking from 44 pulls for 24,000 gems
- 38.89% of unlocking from 49 pulls for 27,000 gems
- Guaranteed epic at 50 pulls for 27,600 gems
Assuming that everyone who tries this keeps going for 11 pulls at a time until they succeed to reach 44 pulls, and then go for singles until they succeed or reach 50, the mean cost of unlocking a Premium item is 22,745 gems
As the count towards the bonus reward resets each time the item is offered, you shouldn’t attempt this unless you have (or are willing to buy) and spend 27,600 gems to unlock the item (and as a bonus, get some progress towards your other items, some upgrade material, and coins) - that’s about 100$ worth of gems 
Of course there will be a few lucky players who will unlock the item for less, but chances are, it won’t be you 
; so unless you enjoy the thrill of gambling away your gems 
, don’t attempt this unless you have the gems to go the distance.