I have a pricelist set to:
compute: formula
Based On: Cost.
Price discount: -15% (in order to add 15% on top of cost)

the rest is left on defaults.

Now it works as long as I add 1 or more units in sale order lines. If I add 0.5 for instance the price is calculated to 0.00. I tried to set rounding to 0.01 on pricelist rules but no success.