This divides into two sections, travelling to and from your start and finish points on the Camino and your daily expenses.
Getting there requires a little planning, the most common start points of St Jean Pied de Port or Pamplona do not have large international airports. I flew to Paris the first time and got an overnight train on sncf to Bayonne; the connection from here to St Jean is easy and very short; alternatively stay on the same train to Pamplona. The second time I drove from Vienna and left my car at Roncesvalles for four weeks. The amount you spend on air travel will to a certain extent depend on how far away from Spain you live and how you get there. See the travel page for airlines and a website called Kayak for information on cheap flights.
Getting home. I think most sensible folk would book their return flights. I didn’t. I wanted some flexibility, I had all summer and did not know if I would finish the Camino before I went to France and Spain. It is expensive to pay for any returns flights from Santiago if they are not booked well in advance. See Kayak, Ryan air, for bus…for train. If you have to be back for a certain time for work, family or anything I would recommend booking your tickets in advance, especially in July and August.
Day to day living can be done very cheaply – alternatively I know of people that stayed in hotels all along the way, that will cost. I stayed almost always in albergues, two nights while on the way I stayed in a pension (bed and breakfast), it cost about €20 each time for one room for myself – this though is along the way – expect to pay a lot more in Santiago, about double for the low end hotels.
Therefore:
Albergue each night 4 – 7€
Evening Meal 7 – 10€
I would start each morning in a cafe having breakfast, about €3 inc coffee. During the rest of the day I would spend about €4 on coffee, tea and cold drinks. Lunch I would make myself, cold meats, cheese, bread, and fruits, about €3. Sometime I would eat in the albergue, cooking with some others, sometimes eating out.
My daily costs were between €20-30. Over the four weeks that I walked I spent about €1000, €250 per week, therefore just over €35 per day on average. I always made sure though that I always had money for any emergencies, (never used or needed). My bank cards worked fine in all the Spanish machines and the same for my credit card.