Wide range zooms don't generally lend themselves very well to extension tubes. Chances are that sharpness may be already tailing off at minimum focus, and to extend it with tubes will degrade it further.
If you want a real macro lens, i.e. be able to reproduce at 1:1 magnification, then there's no such thing as cheap, although some telezooms such as the Tamron 70-300mm have a pseudo-macro facility. These will give ok-ish results but not as good as a dedicated macro lens. The advantage here is you also have a reasonable telezoom.
Another cheap way of getting close is to get an achromatic close-up lens. These screw onto your lens's filter thread.
Raynox DCR-150 or DCR-250 are pretty good and have a universal mount for a range of filter sizes.
These will allow you to get a feel for macro without a great deal of outlay, and with care can give surprisingly good results.