I made my pilgrimage in June/July 2007 and did quite well on about 25 Euro per day. I stayed in albergues however. That incuded coffee stops and occasional evening wines with friends.
You should find previous topics have covered this in some detail with some giving amounts per km but I think per day is much better - the general answer is how long is a piece of string but you should start with accomodation (0 for camping, 5 for refuge or 30 euros per day for hotel), add food and liquid during the day, possibly an evening meal (say 10), then odds and ends along the way (sun cream, plasters, compeed)
Mine was probably closer to 20 per day (refuge but only occasional evening meal) but it's probably better not to carry all your money - I just brought one bank card and one credit card, stopping at an ATM every few days
The albergues are what the Spanish call the 'refugios'. They are nearly never called 'refugios' in Spain. They would be the typical pilgrim accomodations....communal rooms, bunk beds, communal showers, etc. approximately 4 - 7 Euro per night.
I definitely recommend that you spend as many nights as you can in the albergues. You will meet and enjoy so many people there and really get a feel for the fellowship on the camino. On the other hand you will 'sleep' with quite a few people in your room in close quarters. If snoring disturbs you bring ear plugs but remember you will be more physically tired than usual so may sleep better than ever before.
If there are days when you do want a private room look for a hostal (a small private hotel room, usually basic but very clean with your own bathroom and even a TV). Other options are pensiones (usually a private room but may be a shared bathroom) or a newer option called 'casa rural' (often quite lovely, newer, private rooms and generally a private bathroom). Breakfast may be included with any of the options. It is nice to be alone in your own bathtub once in a while.
In my opinion is "possible" to walk for 25 euros a day. Possible means: being carefully with your spending. So, ¿necessary? Convenient at least one menu a day (9 euros) another one is possible (¿8 euros?) 5-6 euros albergue and 2 euros for breakfast.
It's possible to reduce this spending, dinner into the albergue, or having just a sandwich on lunchtime (buying the necessary in a supermarket)
In Navarra (first part of the Camino) is everything more expensive, so it's possible to need more than 25 euros.
And sometimes is a good idea to spend a bit more because most cheat menus doesn't include the best specialities to taste (you know, Spain is weel known for it's good meals and cuisine specialities)
The ball park figure of the Camino is widely given as 1Euro/km so the 750km walk over a 30 day period averages to 25Euro/day if a total of 750Euros is allocated for the walk, as advised by our regular contributor Javier. A fairly detailed cost of every item, meals, accommodation, drinks, etc on the Camino is on our ever informative Sil's posting.
Overseas pilgrims can take, as mooted by several contributors on this forum, two years of planning. We saved and earned one extra Euro/day over the 2 year planning period and we were able to double our available funds for the Camino from 25Euro/day/person to 100Euro/day for me and my wife. We never got to spend that amount, but it gave us much more options .... never had to sleep on the floor or take a cold shower.
My posting tries to give a holistic view of the impending journey, and to try to put things in perspective. There is the initial cost of the equipment, cost of the travel between the home and starting point, loss of income during the walk, so the saving of staying in the cheapest lodgings and scrounging for food plays only a small part of the overall cost.
Leave the cheap end of the travels to the people who are after a cheap holiday, willingness to spend a few extra Euros can open an avenue to many things. On security take the advise of the other posters and keep minimal cash with you, we were robbed of one set of our ATM and credit card but had a spare. Be flexible and enjoy the experience - obviously the more dinero the more the flexibility.