Magic item creation ring of permanent Shield

[Spatzimaus]

See, arguments like this are why I don't allow many continuous-effect items any more.

These guys are right. The DMG rules are guidelines, and it's always fair to just say no. The Sage, a while back, said that continuous effects should be priced more on their effect than as spells. So, you can't substitute an item with continuous Bull's Strength for a belt of giant strength.
In this case, IF you allowed it, I'd say 100k. 122.5*0.75 for the cover bonus, plus a little extra for the Magic Missile protection. Unlike true cover, Shield doesn't give a Reflex bonus or prevent AoOs.

Or, have it be like Boots of Speed, where they get a limited number of rounds per day for it.

 

[dcollins]

Give them a brooch of shielding. Cost 1500 gp. This is what they get when using a shield spell to craft a wonderous items.

That's a nice observation. I need to remember that the next time the "continuous shield item" thread pops up.