Doesn't have to be a lot. 20 or 25 AP is totally fine. Hell, even 10 AP would be nice. 
Note that I do not think that this item is unbalanced or anything; I think the item is workable as it is. But I'd love to be able to build this as an AP mage in some sort of semi-troll build, just for the scalings.
I actually agree with this, mostly because there's no point in having hybrid stat champions anymore. There's a lot of champions (like Vi and Lee Sin), who have the AD and AP scaling but the only items that have those mixed stats are items that are terrible on them (Gunblade, Rageblade). I mean, you aren't going to build a Deathcap on Lee Sin just so you can have a fat W shield.
So, what ingredient will you add to make it 20Ap? I think an Amplifying Tome is fine, but then its cost will be increased to 4168. So some of the stats like Atk speed should be reduced in order to have AP in it
