Can the incrementer in a portal be based on a field?

[Mathias]

I have a set of items I'm adding to my game system - some have variable costs. I have the code written to switch portals between a display for those with a fixed cost and an incrementer for those with a variable cost. The problem is that in one cast, the incrementer I need is 2, not 1 (the thing allows costs of 2, 4, or 6). I had already writtten a field (with defvalue="1") into this thing in order to allow the user to set a different increment when needed, but when I tried to set;

<incrementer
field="valCost"
interval="valInc"
</incrementer>

it gave me an error. For now, I'm going to write a second incrementer, with increment="2", and add an if/then to switch between them.