It is 5 + item casterlevel.
For any items already in the book, it's just 5 plus the listed caster level, plus another 5 for each item in the Requirements section the caster skips.
If you're talking about how to set the caster level for home brew items, you can basically set it however you want between the minimum caster level needed to cast the highest level effect on the item and either 20 or your own caster level depending on how you interpret various details.
Edit: This works for any magic items, not just wondrous items. For weapons and armor the base enhancement bonus is one effect and everything else is treated as individual effects.