Still here folks...
Classic B/X Prime Requisite perks are a +5% XP modifier for scores 13-15 and a +10% XP modifier for scores 16-18.
I've always completely ignored this and never gamed with anyone that didn't.
The last thing we wanted to do was bust out the calculator when dishing out XP.
Try something like this instead. It's easier, more fun, and gives your character an equivalent and tangible edge during actual play as opposed to an end of the night calculation.
On the following tables, you get 1 roll if your Prime Requisite is 13-15, and 2 rolls if it is 16-18. These rolls take place at character creation and never again. These charts reflect positive Prime Requisites, but, can easily be reversed for negative scores. If your Prime Requisite increases or decreases during play, add or subtract these bonuses as needed.
Elves can roll on either the Fighter or Magic-User tables.
Roll 1d4...
Fighters/Dwarves/Halflings/Elves
1. +1 to-hit
2. +1 to armor class
3. +2 hit points
4. +1 to damage
Clerics
1. +1 to turning undead
2. +2 hit points healed with curing spells
3. +1 hit point
4. bonus 1st level spell (when available)
Magic-Users/Elves
1. bonus 1st level spell
2. +1 save vs. spells
3. targets are -1 save against your spells
4. bonus 2nd level spell (when available)
Thieves
1. +5% to all percentile thief skills
2. +1 to-hit with back-stab
3. +2 to back-stab damage
4. +1 to a single saving throw category