Accommodation ate the biggest share, about $28 a day for a private room in a guesthouse near Alfama, booked six weeks out. Food came in around $18 a day by eating lunch at tascas (the workday set menu, not the tourist menu) and cooking breakfast in the guesthouse kitchen.
Transit passes cost $1.65 a day using the Viva Viagem card, and the remaining $12 covered museum entries, a day trip to Sintra by train, and one splurge dinner near the end.
The number that surprised me most: skipping one sit-down dinner a day for a grocery-store picnic saved almost exactly what the Sintra day trip cost.