A budget entree into macro would be to use a reversal adapter to stack a 2nd hand manual focus lens in front of your existing lens (e.g. a kit 14-42). I've posted some pictures taken with this setup here
http://www.clubsnap.com/forums/showthread.php?p=4978912#post4978912
An advantage of this set-up is that you retain automatic aperture control and a limited range of autofocus. Disadvantage is that working distance is short and primary focus will still be by moving the camera back and forth. Adaptor costs ~$10-20 and lens would cost $100-$150.
But I must say I'm much happier now that I have a Vivitar 90/2.8 macro lens. Yes, it's manual focus and manual aperture, but the image quality is good and most importantly, I actually have decent working distance now ! Having the working distance also solved a lot of the lighting problems I had earlier because now the lens is not jammed right up to the subject. I paid ~US$200 for it on ebay but I've seen similar lens going for less. Depends on your luck and patience in bidding. OM adapter is ~$20.
If you can find a used sigma 105 within your budget, go for it, but if not, you can try using manual lenses.