How inexpensive? If you can live with a painted bookshelf, then use MDF for the carcase. A full sheet of 3/4" MDF is probably about $27. From this you can create a very stable and well-constructed case, including shelves. If you make a simple shelf pin jig, you can use shelf pins to have adjustable shelves. The design is up to you. My point is, you can use MDF to make any joint you want (dadoes, rabbets, pocket hole joints, etc.).
To this you can add wood moldings and a wood face frame. I would suggest poplar, because it is fairly durable and cheap. After you attach the face frame and moldings you can putty the nail holes or whatever and then prime and paint and you're good to go. Adding an edge strip of hardwood to the MDF shelves is a good idea to increase their capacity and avoid sagging. If you can, make the shelves out of hardwood, as well as add the front-edge strip.
Anyway, you could make a pretty good bookcase for probably around $50, maybe less.
Mike