Hi,
I might have misinterpreted your question, but a 50mm lens is half the focal length, and therefore has twice as wide an angle of view as a 100mm lens. So if you take a picture of a subject with both lenses then there will be twice as much information in the photo taken with the 50mm because of the wider angle of view.
If both lenses are macro then we need to know what magnification each lens has in order to compare the two. Assuming that the lenses have comparable magnification then the difference will be that you can be farther away from the subject with a 100mm lens to get the same amount of information in the photo. If you take photos of bugs for example, you really have to get very close to the bug to use the macro function, maybe inches away. This would interfere with lighting and maybe even disturb the bug. With the 100mm lens you would be further away and get the same angle of view. If you were taking macro shots of venomous snakes you could get even further away using a 200mm macro lens and still get a good macro shot. That would keep you out of the hospital - mostly.
50mm lens are the easiest lenses to manufacture, therefore they are generally the least expensive of all lenses. The longer focal length lenses are much more difficult to make, so a true macro 100mm will be relatively expensive. This will depend on other characteristics of the lens though, such as maximum aperture, whether the lens has fancy things like image stabilization, and of course the maximum magnification.
Cheers,