This is the most expensive place in Chile to be a backpacker — two concessionaires hold the beds inside the park and everything you eat there was driven or carried in — and the one decision that controls the budget is whether you sleep inside the park or commute from town.
Sleep in Puerto Natales, day-trip the park
A dorm in town costs a fraction of a bed inside the park, and Bus Sur runs Puerto Natales–Torres del Paine daily at 12:00 with other operators covering the morning departures. You give up sunrise at the towers and halve the cost of a week.
What a day costs
Park entry is bought in advance through CONAF's Pases Parques site and is priced by season and by nationality, so price it before you plan the trip rather than after. Inside the park a refugio bed with meals is several times a Puerto Natales dorm; a town day with your own cooking plus a bus fare is the cheapest kind of day this region offers.
Camp rather than take a refugio bed
Every sector on the W has camping alongside the refugio, run by the same two concessionaires, and pitching your own tent is by a clear margin the cheapest bed inside the park. It still has to be booked before you set out — CONAF registers W walkers at control points and your reservations are what get checked.
Erratic Rock, Puerto Natales
Hostel and guide service running since 2004 that also rents trekking equipment and puts on information seminars for people about to walk the W or the O. Renting a tent, bag and mat here beats buying kit you will use once.
Winter is cheaper, and restricted
Outside the December to February peak, beds and buses drop sharply — but from 27 April to 15 September CONAF requires an authorised guide on the Base de las Torres trail, which changes the arithmetic. Weigh the guide fee against the saving before you plan an off-season trek around price.