I've found that those half width jewel cases will actually hold two cd's if you don't mind the second cd stacked on the first and touching the jewel case. Should be ok as long as you're not moving them around. A box of 200 cases from Costo with shelves now holds 400 cd's.
Though, I found out the hard way if you're using super cheap cdr's the top reflective layer will occasionally stick to the jewel case and becomes damaged when you unstick the cd. Solution was to put the cd's in top side down.
When I run out of cases I just stack them back on the spindles.