Never use the magic item construction guidelines to make permanent items based on spell effect if you can also use another chart that would produce a more costly result. You're bound to run into something awful. If you use those pricing guidelines to make Shield permanent, how about True Strike? It's only 1st level, too! So cheap!
Your shield spell provides an AC bonus. It's not an armor bonus, so it's a bit more expensive than 16,000 gp (4x4x1,000). Instead, it will stack with an armor bonus, so that would make it roughly on par with an Amulet of Natural Armor (natural armor bonus, bonus squared x 2,000), or a Ring of Protection (bonus squared x 2,000, as well). However, since natural armor bonuses and deflection bonuses can't be obtained by simple mundane items, while shield bonuses can, this might be a bit too expensive. I'd probably use something like "bonus squared x 1,500 gp" to come up with a final price. So your permanent Shield item would cost you 24,000 gp.
But wait! The Shield spell also protects from Magic Missiles! So we'd have to include that effect in our cost calculation. A Brooch of Shielding can absorb 101 points of MM damage and costs 1,500 gp. Your item, however, would lack such a limit to the amount of MMs it can absorb. So let's at least double that price. And then double it again because we're putting two effects into one item, leaving us with 6,000 gp additional cost.
An item that produces a constant effect that works like the Shield spell would be estimated to cost 30,000 gp (market price) by this calculation. I think it's a fair price to ask, if a bit on the expensive side. As a DM I'd probably be OK to reduce that price by 10 to 20 per cent.