A Dressmaker dress sells for its material cost plus the labour it took, multiplied by a quality bonus and then by a prestige bonus. The bonuses are the part that surprises people: Quality 50 is a straight 1.00× and leaves the base value alone, while Quality 80 raises it by 30% and Quality 100 by 50%. Prestige adds one percent per level on top of that, so the same dress is worth noticeably more at prestige 20 than at prestige 5. Because the game settles its money in whole coins at each step, a sale price is not the same as multiplying both percentages together at the end.
The other half of the number is the quality score itself. It comes from grain placement, fit and sewing accuracy, weighted 30/20/50 when the dress was cut to a customer's measurements and 40/60 when it was not, because fit cannot be scored if nobody was measured. That is why two dresses with identical cloth and trim can sell for different amounts, and why the quality breakdown on this page separates the three inputs rather than showing one figure. A sale price calculator is only useful if it can tell you which of them to work on next, so the what-if block prices your own dress at a higher quality and at a higher prestige level and reports both moves in coins.
A sale price is half the answer, because the cloth and trim had to be bought first. What a dress adds to the purse is its sale price minus its material bill, and that remainder is what a money goal is saved out of: a purse, the price of the thing being saved for, and the net gain of a build are enough to say roughly how many dresses are left to make. Labour sits outside that subtraction — it raises what the dress is worth without being Coin spent at the bench — so a build can sell for more than its materials and still be the wrong one to repeat if the silk was dear.
Sale prices measured against data build 25482629, which lists no published change to the sale-price chain.