Well, you can use hardboard underneath foamcore flooring for added support. Walmart and Target sell 20x30 sheets of foamcore for an okay price... but if you have a Hobby Lobby around they go on sale for 99 cents a sheet every so often.

If its for display then I don't know a good way to make sectional walls for cheap... but if its a limited use set for pictures then just taping walls together or making back support braces is the easiest way I've found to have movable walls. Make a set of right angle braces with balsa wood and attach them behind the walls... they'll keep the wall up and make them modular at the same time.