You'll struggle to get auto tubes for that price.
Be wary of cheapo tubes, they don't have electrical contacts so you loose auto focus and, more importantly, aperture control so you have to either shoot macro at f/1.8 (useless really) or resort to holding the DoF preview button at the required aperture while dismounting the lens.
Better to save for auto-tubes.
Another option might be adding a raynox DCR 150 or 250 supplementary lens. Great quality little lenses, but not as flexible as extension tubes, and they're more designed for tele photo lenses.