Aaaww, too bad, really thought it was just a thing of a bit more arithmetics... something computers are much better with than human beings.
Thanks anyway for the explanations, hope I'll be able to pass them on as well when asked...
nicolas wrote: An easiest solution (although more work for you) is to create a custom product field where you would enter the price that you want to be used for the filtering in each product and then have your filter apply on that custom field instead of the price.
I agree that it's a valid and practical workaround for a small to mid-sized store with the occasional discount. I'm already using custom fields as cheats for other, simpler purposes, but for a market thingy with lots of products and vendors, and ever-changing discounts I feel it doesn't want to be managed manually.
I'll see how far I get without the extra math.