On your own, plan on €140–190 a day. That covers a bed in a hut with dinner and breakfast, a lunch on the trail and, on some days, a bus or a lift. Over the usual 10–11 days it adds up to about €1,600–2,000 per person, before you pay to reach the start.
The range is wide for a reason: the country you sleep in moves the price more than anything else. A night with half board costs €60–85 in Italy, €75–100 in France and €100–130 in Switzerland. How many buses and lifts you take is the second lever.