I find the very concept of a stat boosting manual to be extremely bizarre.
Think about it.
A +5 stat boost manual costs 137,000 GP to purchase, or 6,250 GP + 25,500 XP to craft.
Only the very wealthy could afford them, and only the most desperate for gold spell caster should even consider making one for someone else.
It's 25,500 XP!!!
That's enough to go from level 25 to level 26 (and almost 26 to 27).
What spell caster in his right mind would craft an item that lowers him 1+ levels in XP for anyone else? For himself, sure. But for someone else? He should have to be extremely desperate for the gold.
And once crafted, which person (purchaser or spell caster) wouldn't use it immediately? What good is crafting such an item and then not using it? Even if someone else stole the item, why wouldn't he use it immediately?
And, even if someone did commission such an item, it should take the spell caster a long time to acquire the XP (and at the same time, not use it for leveling up) with the core rules.
This type of item should NEVER be found in a treasure vault.
The XP and the item craft rules just do not mesh at all on these items.