The longer the focal length the more expensive the lens. The minimum focus distance ranges from 7.5" (50mm) to 15" (150mm).
Personally, I have the Sigma 70mm macro and am really impressed with it. For $429, it does a nice job and it's sharp as a tack. Plus the focal length makes it useful for portraits as well. Generally, if you are shooting a skiddish subject, you want a longer focal length like the 150 to give things like butterflies a little breathing room.