I have purchased a Tamron 90mm f/2.8 SP AF Macro lens (not the DI model) for less than $225 on Ebay (including postage). It has turned out to be an excellent macro and general purpose lens and competes very well with the Canon 100mm f/2.8 Macro. In fact in some categories, the Tamron surpasses the Canon... although in most categories, they are a dead heat.
http://www.photo.net/equipment/canon/can-tam-macro/
The purchase price wasn't a fluke. These Tamron lenses usually sell in the neighborhood of $200-225 Used.
The lens has beautiful bokeh and has been called the "Macro Portrait Lens". It is easy and fun to use and the images appear to be extremelt sharp and crisp.
I am still getting acquainted with all the facets of this lens. It has been many years since I owned a dedicated macro lens. I have seen some absolutely wonderful macro images shot with this lens.
The reason I posted this is to let you know that there are other viable options than the Canon 100mm macro.