Not much extra expense. Especially if cow Fluffies are a thing in that universe.
Fluffies would be cheap as hell to raise. Even cheaper if you have a garden, and use @Chikahiro’s idea of fermenting food for healthy reuse of their shit as second meals.
The only issue is them needing constant care. Ones for sale would need not only gentle discipline, but no exposure to stress. A source of milk that isn’t a tormented limbless and eyeless Fluffy would no doubt be better. Since that payoff would be worth it for fulltime work he could just keep regular milking Fluffies. Stimulate lactation manually by rewarding them for dry suckling each other until it begins, then reward them again for using the milking machine. To boost nutrition and reduce cost, use spaghetti-like vegetables.