As Charles points out, a key difference between 'macro' lenses and normal ones is a flat plane of focus (given the way optics work, the plane of focus is normally a section of a sphere). This makes macro lenses expensive.

For anyone with a Nikon DSLR body, I'd suggest looking at the Nikon/Nikkor 28-105 1/3.5-4.5 D series lens.

It's switchable between normal minimum focus distance and macro and has a flat-ish plane of focus. I see them on eBay for around $75, making it an inexpensive macro(ish) lens.