The hospedaje in Santa Cruz de los Seros, about 5km away from San Juan, is a good base as I do not think that there is any pilgrim accommodation there-- the closest albergue is the highly-praise one in Santa Celia. The hotel at the newer monastery (about a km beyond the old one, which is in this photograph) is rather pricey -- I can't find current prices on their site but I recall it was well over 70E in 2007 and I don't know if they would give a pilgrim rate.
The monastery is absolutely incredible, The carvings of the maestro of San Juan are spectacular and we could not have understood them before Picasso. The monastery itself is like something out of a Maxfield Parrish painting and the above photo gives you a good picture. I rank it as one of my top three experiences in Spain.
This monastery is so intriguing to me that I'm thinking of staying over for a day in the place oursonpolaire is recommending or in Santa Celia to take it in properly. The place in Santa Cruz de los Seros looks amazing... not quite pilgrim prices but perhaps a treat along the way