Well Wow haha. The way you worded your original post I think that almost all of us thought that you were talking about private rooms and restaurants each night. There was no mention of you looking to sleep in basic environs.
The least expensive and often the most rewarding options are donativos and municipal albergues. Remember donativo does NOT mean free. You should leave a minimum donations of what a municipal albergue would charge and if they offer a communal meal you should also leave at least 10E. If they offer both you should leave at least 20-22E. Some even offer breakfast. 3E would be the minimum for that.
You can also prepare dinner at many albergues. (Not the municipal Xuntas in Galicia). Most have a partial kitchen and many have a full kitchen. When you arrive check to see what pots and pans and spices etc an albergue may have before you go to the store to buy food. A partial kitchen will most likely be a refrigerator and microwave.
No one source has all available places to stay. You should always look at Gronze.com and there are also two very good apps called Buen Camino and
Wise Pilgrim. They will all give you a description of the albergues and tell you if they are monasteries or donativos etc. There are some amazing donativos that have really only the most basic of basic services but reward you in many other ways. For example these are some wonderful and famous and as basic as they come albergues:
Información del albergue de peregrinos Albergue parroquial San Juan Bautista, en Grañón: Precio, plazas, características, etc...
www.gronze.com
Información del albergue de peregrinos Albergue Hospital de peregrinos de San Antón, en Convento de San Antón: Precio, plazas, características, etc...
www.gronze.com
Información del albergue de peregrinos Refugio Gaucelmo, en Rabanal del Camino: Precio, plazas, características, etc...
www.gronze.com
There are any others