Also, check the minimum focus distance on Sigma on the Best Buy link, I bet its pretty long - like 3 feet?
For "el cheapo" macro shooting, what about a $100 50mm AF 1.8 with a $30 set of 52mm close-up lens attachments? The neat thing about that is the prime 50mm is a great lens to use without the close-up attachments.
I have a 60mm micro, the above 50mm with close-ups, and I still get my best shots with my 18-200 VR - probably because its more forgiving of my poor macro skills., it is not a macro of course, but the minimum focusing distance is only 18", and I can use an on-camera flash at that distance (as opposed to a ring light or my SB-600 on a sync cord for the other two lenses). The resulting images are crisp enough on my D300, that I can crop them to "macro size" and just go with that. The benefit is that the 18-200 also serves as my main lens for just about every other shot I need to make.
Jay