Technically, it would be equivalent to some amount of AP, depending on the AP ratio of the shield/heal. A 10% increase affects the base value, so increasing the shield by the same amount with only AP might get to be a bit difficult.
I'll use Janna as an example. At rank 5, her shield is 210 + 70% AP. If she has 100 AP, she will shield for 280. A 10% increase on that is 28, resulting in a 308 shield. In order to increase the shield by the same amount, she would need 40 AP. That doesn't sound too bad, but what happens when she has more AP?
I made a couple of handy graphs in Excel to help me. I figured supports don't get too much AP, so I ran the numbers out to 350 AP in increments of 10 AP. It's easy to show how big Janna's shield would get with just the AP bonus (270+.7AP) and then to add a 10% or 40% bonus to that to replicate the items (total1.1 or 1.4 respectively).
With that, I found Janna's shield can get up to 515 with 350 AP, 566 with 350 AP and 10% boost, and 721 with 350 AP and 40% boost. And no, I didn't bother looking at items to see if it was possible to get 350 AP (or more) with a reasonable support build. Overall, these numbers aren't surprising, since they're just 10% and 40% higher, respectively. What is interesting is the question OP asks, "what is the AP equivalent?"
I was able to determine how much extra AP Janna would need to hit the same shield amount ( ((total-270)/.7)-AP ) and got some interesting numbers. a 10% boost is equivalent to 38.5 - 73.5 depending on AP. This means that Janna with 0 AP and one support item that grants a shield increase shields for the same amount a Janna with 38 AP shields for. A Janna with 350 AP and a supporty item shields for the same amount a Janna with 423.5 AP shields for.
The difference becomes even more with all three items. A Janna with all three gains 154 - 294 equivalent AP, resulting in a shield of equivalent strength to a Janna with 594 AP.
Granted, this only affects shield strength and heal amount. A Janna with 594 AP would also be providing additional damage and a greater amount of utility (shield grants more AD, slow slows more, etc). The flip side is these items don't just give Janna a shield/heal boost, but also provide additional effects, such as increasing the attack speed of her shield target, etc.