I don't see why it should be more expensive, no. I'd leave it as is. But my question would be, what would it even be? Head? Face? Shoulders? Vest? Yes it says cloak, but for what it does, is there really a slot dedicated to an item like that?
The equations for estimating market value is just a starting point... just an estimate. The last paragraph on page 282 of the DMG sums up pricing magic items. I won't retype the whole thing, but it's all a judgement call by the DM. The item should be as expensive as it is worth... If you were a vampire, how much would you be willing to pay for something like this?
When adjudicating the cost for custom items with my players, I advise that a price that sounds too good to be true... is too good. When a price is high enough that it makes you really have to weigh the pros and cons, it's probably in the right ball park.
One of the other guidelines from that DMG paragraph is to use existing items to guage the market value of a new item.
From Libris Mortis, page 78: Goggles of Day (4,500gp) "A bonus side effect allows vampires a full-round action prior to dissolution when confronted with sunlight, as opposed to just a partial action."
Ok, granted, this is a side-effect. For a vampire though, that's the main selling point. I'd pay ten times that for full immunity.
(The_Mad_Linguist, I think these are the droids you were looking for)