As of May, 15, 2013 this achievement is finally obtainable and no longer glitched. Upon launching the game, download the update and it should unlock once the 50 prisoners are bought.
Note: It appears that if you already have 50 prisoners upon downloading the patch, in order to get the achievement, you need to purchase another head and it should unlock. For those who already had all of the prisoners upon downloading the patch, you will be unable to buy any more heads, but you can still get the achievement. Just trade a head away to a random or a friend then re-buy it from the head dispenser and it should unlock.
You unlock this achievement by purchasing any 50 prisoners from the gift shop. The
way that the gem pricing works is that the base value, with only the four default prisoner heads that you are given upon starting up the game, is 10 gems per prisoner. Once you reach 30 prisoners purchased in a specific head shape, the base price increases by 5 gems for that specific head shape. Once you reach 60 prisoners purchased of a specific head shape the price increases by another 5 gems for that specific head shape. Now once you own 60 prisoners in all of the head shapes combined, the base value of all prisoners increases by 5 gems. And continues to increase by 5 more gems at 120 prisoners purchased, 180 prisoners purchased and 240 prisoners purchased. With these values, the amount of gems needed to unlock 50 prisoners is going to vary, depending on whether or not you buy from the same head shapes or not. If you buy 50 prisoners, all from one head shape, it will take 29 heads * 10 gems (29 because you already have a default head of each shape) + 20 heads * 15 gems = 590 gems. The minimum gems you are going to spend on this achievement is 500 gems. Because you are buying 50 prisoners at a minimum of 10 gems per prisoner, 50 * 10 is 500 gems. So depending on your buying pattern, this achievement costs 500-590 gems.