So for instance, if you have a stock of R2 mats in your warbank,
it will just use those instead of requiring some supposedly 1c cheaper R1 mat :)

An even better option but that would require a little more work would be to have some input box with a "material % value tolerance"
So as long as you have the required mats regardless of rank, if the price difference is below that % threshold, it just use what you have.
only excluding craft if optimal mats have a noticeable price difference compared to the mats that you currently have.

That comes from me trying to purchase some R1 mats and finding out that the little discrepancy between the TSM price and the actual price on the AH made the R1 slightly more expensive than the R2.

In such case, the only way to have the craftqueue to work is to purchase the more expensive R1 material regardless which is a bit counterproductive in gold and time spend on the matter :)