Here is a breakdown of the numbers of pearls you get from an item bought at Granny base on her selling price.
(Number of melting slot unlocked / cost of one pearl / cost of one pearl during Blacksmith event)
1 slot 250k/pearl 50k/pearl
2 slot 200k/pearl 40k/pearl
3 slot 167k/Pearl 33k/pearl
4 slot 125k/pearl 25k/pearl
5 slot 100k/pearl 20/pearl
6 slot 83k/pearl 17k/pearl
7 slot 63k/pearl 13k/pearl
All 8 slot 42k/pearl 8k/pearl
Exemple : You have 4 melting slot and buy an item at 1 500 000 gold from Granny. You will get 12 pearls if you melt this item (or 60 during the event). If you get an extra slot (#5) you would get 15 pearls (75 during event).
P.S. Your item value is 10% of Granny’s selling price.