Selecting the prbability/gem of getting goodieshas to balance two effects:
- low odds discourage spending because people see too little chance of getting the goods
- high odds discourage spending because people quickly get what they want
I am certain the SGG business analysts study these countervailing effects very closely and tweak the effective cost of gems accordingly (using special offers).
Some advice from someone who has spent too much on this game:
- don’t buy gems from the shop. Ever. Only buy gems when bundled with other useful items, particularly 4* ascension mats
- set a budget and stick to it
- read What exactly is a Gacha? (Read before posting things about "unfair pulls")