I use old carbon arrows in bigger planes but they are just too heavy in the little jobs. I like the 1mm by 3 mm flat carbon spars sold by depronusa, about the same price as the arrow but much better suited for the purpose. cut a slot in the wing or fuse and glue it in. very stiff with very little weight penalty. I know what you mean about the local hobby store tubes being high but I never seen any for 30 bucks!

Jason