Done with this exercise too. Initially I got the Total Cost field values incorrect, then I went to the formula tool and checked the formula used for getting subtotal and I removed the brackets I have used in the formula, after removing brackets and re running the workflow i got exact correct answer.
my initial formula was as- ([Unit_Price]*[Quantity_ordered_new])-([Discount]+[Shipping_Cost])
My corrected formula was- [Unit_Price]*[Quantity_ordered_new]-[Discount]+[Shipping_Cost]
I think this might be because how the business rule was intended for this question. Good to learn !
