If you glance through the dmg, you'll not many rings have caster levels below 12. However, rings require caster level 12 just to make.
Now a caster can voluntarily lower his caster level, but why would he in this case? The cost doesn't increase with caster level when it comes to rings.
Now a caster can voluntarily lower his caster level, but why would he in this case? The cost doesn't increase with caster level when it comes to rings.