The town is cheap and the tour is not, so the single number that decides your budget is what you pay per person for a seat in a 4x4 and how many people are squeezed in beside you.
What a day costs
In town your only real costs are a bed and market food, both among the cheapest in Bolivia. The fixed extra nobody warns you about is the Reserva Eduardo Avaroa entrance fee — Bs 150 for foreign visitors, Bs 30 for Bolivians, cash at the gate and not included in almost any tour price.
Carry cash, in bolivianos
Uyuni has only a handful of ATMs, card payment is rare outside the larger agencies and hotels, and there is nothing at all once you leave for the circuit. Draw what you need in Oruro, Potosí or La Paz and carry small notes.
The one-way finish at Hito Cajón
Most three-day circuits will drop you at the Bolivia–Chile border post at Hito Cajón rather than driving back to Uyuni, with an onward transfer into San Pedro de Atacama. It saves a day, costs extra and is booked as an add-on, so settle it before you pay.
Sleeping on the circuit
Circuit nights are in basic village lodgings and salt hostels around San Juan and inside the reserve — shared rooms, no heating at over 4,000m, hot showers charged separately. Sleeping bags rent cheaply in Uyuni and are worth taking.
Altitude is the hidden cost
The circuit sleeps above 4,000m and reaches close to 5,000m at the geysers, which is where people lose a day of a tour they have already paid for. Arrive from La Paz, Potosí or San Pedro rather than straight from sea level, and give yourself a night in Uyuni first.