The dramatic rise in property prices over the past several years has lead to high competition for available rental properties. Monthly rent for a 1 bedroom furnished flat starts at €600/month in Madrid and Barcelona. A cheaper option is to look for a room in a shared apartment (piso compartido), which may start around €300/month in the major cities.
May to July is the best time for flat hunting; August can be difficult as people are on holiday. September tends to be the worst month as people return from vacation and students start a new year. The housing market can also be slow over the Christmas period.
Finding a good apartment at the right price before you arrive in Spain is difficult. Landlords prefer to meet tenants before signing a contract and the wide range in quality means it is not advisable to commit to rent a flat "unseen". If you require your living arrangements to be organized before you arrive, you can use an apartment rental service. However, these do tend to be expensive.