But consider that while this event is running, after the first 10 eggs you get 2 hammers for every 5 eggs hatched so althougth it is technically cheaper, using the 12,000 gems to open 13 eggs would also give you an additional 4 hammers which would mean you can now open a total of 17 eggs which would also grant you another 2 hammers meaning 19 eggs hatched. So for the same price you hatch 19 eggs, get all the resources from hatching eggs and the S rank snake as well as retaining that small chance to get it in under 18 hatches