STAB multiplies damage dealt by 50%, or 1.5x.
STAB technically doesn't add base power, but rather multiplies the damage dealt (though multiplying the base power by the 50% STAB provides still gives a rough estimate on how much it is worth).
So technically taking a move like Earthquake (100 base power) and increasing to 150 base power would not be the correct way of calculating it; first you determine how much damage the attack would do taking into account the Attacking stat, Defence stat, the moves initial base power (which in this case is 100), etc. and then adding boosts like STAB, boost from items, moves like Swords Dance, etc. on at the end.
For example:
252+ Atk Abomasnow Ice Shard (using the correct formula) vs. 4 HP / 0 Def Garchomp: 240-288 damage (67 - 80.4% of max HP)
252+ Atk Abomasnow Ice Shard (adding STAB to the base power rather than at the end) vs. 4 HP / 0 Def Garchomp: 240-284 damage (67 - 79.3% of max HP)
As you can see, the difference is only 4 damage in this example, but they aren't exactly the same.
Credit to Pokemon Showdown! for the damage calculations
