Greenfield
Adventurer
A Ring of Force Shield is already in the book at 8,500 gp. It's a +2 AC bonus, and doesn't automatically block Magic Missile spells.
This is where the other rule of magic item creation comes into play: Find an item of comparable power in the book, and use that item's price as a starting point.
A normal Ring of Protection +4 would cost 32,000 gp, and still wouldn't block Magic Missile spells.
So yeah, you're right, your approach to the ring is much too cheap.
As a general rule, if you want magic protection, lots of small items are cheaper than a single big bonus item, they just use more slots to do the job.
This is where the other rule of magic item creation comes into play: Find an item of comparable power in the book, and use that item's price as a starting point.
A normal Ring of Protection +4 would cost 32,000 gp, and still wouldn't block Magic Missile spells.
So yeah, you're right, your approach to the ring is much too cheap.
As a general rule, if you want magic protection, lots of small items are cheaper than a single big bonus item, they just use more slots to do the job.