Pensions or Hostels
Setting aside free camping (which I do not think should be your first choice, seeing as driving a motorbike after sleeping on the floor is something that requires quite a bit of stamina) pensions in Spain provide accommodation at cheaper rates than hostels. Of course this comes with several tradeoffs, including old worn out rooms, lumpy beds, shared bathroom with the entire building or floor, no hot water after a certain hour, reduced safety (no night reception) etc. depending on the establishment of course.
All of this is from my personal experience. Indeed I have slept in a pension in Cadiz in a room whose door wouldn't lock, in an uncomfortable bed. And I woke up with a handful of people outside my room peeking in presumably to check out what was available to steal. I have also slept in a pension in Zaragoza which was nothing more than the extension of an old woman's flat. I had a bed and shared bathroom and everything went great. The point is that pensions have lower standards than hostels, and this is something to keep in mind.
The Spanish equivalent of the French Gîtes de France are the Casas Rurales. These are often manors which are rented to large groups or families, and have all the amenities you can think of - kitchen, pool, etc. I mention then here anyway because it is indeed possible to rent a room for cheap in a Casa Rural, especially during low season, and when the house isn't occupied by anyone. I did this a couple times in small villages that had no hotels let alone pensions. Just find the Casa (they are usually marked with street signs like the Gîtes) and call the owner's number you will usually find on the front door.
For more information see this website detailing the meanders of accommodation in Spain.