Final answer:
The total contribution to GDP in the given scenario is $90,000, which is the sum of the values added by the farmer, miller, and bakery at each respective stage of production.
Step-by-step explanation:
The total contribution to gross domestic product (GDP) is calculated by considering the value added at each stage of production. Farmer Bill sells wheat to the miller for $50,000, the miller sells flour to the bakery for $75,000, and the bakery sells bread for $90,000. To avoid double counting, we consider the value added at each stage: the miller added $25,000 (${75,000 - $50,000}) and the bakery added $15,000 (${90,000 - $75,000}). Therefore, the total contribution to GDP is the sum of these values: ${50,000 + $25,000 + $15,000}, which equals ${90,000}.