The selling price cannot be negative (only 0 if it negative), but I could not make a suitable formula due to restrictions (cannot be divided by net price). What can you advise in this case?

If you take a look at the first part of the formula posted second time, then there is a difference compared with Excel.
Excel: (ABS(B2)+B2)/2
UI: ABS((price) + price)/2
First will give you the value of the price, if price is positive, or 0 (zero) if price is negative
Second will always give you the absolute value of the price, no matter if the price is positive or negative
For the UI formula, move the first parenthesis after the ABS in front of the ABS function.
Must be something like this: