OK, I went to take a nap and I think I have this whole business figured out.
Expensive EOS FD Macro: Ebay non-automatic EOS ext tube ($20), Ebay EOS/FD Adapter ($30) + FD Macro 50/3.5 ($100) = $160 before shipping
Cheaper EOS FD Macro: Ext tube ($20), EOS/FD adapter ($30), Canon FD 50/1.8 ($20?), that's $70, now down to the price of a Nifty Fifty
Cheapest EOS FD Macro - I just thought of this and I think some of you are really gonna dig it: Ext tube ($20), Canon FD 50/1.8 ($20), elbow grease*
* - Get a Canon EOS BODY CAP, or if one comes with your chosen tubes you are ALL set. This should cost you all of $5. You may already have a toolkit of various kinds, get some instrument and cut a hole in the body cap, you are going to McGyver the FD lens into fitting into the EOS body cap. Alignment is important, the lens should line up properly, I can't predict what will happen if it does not, so check your alignment before you hot-glue this contraption into place.
You now have a FD lens sitting on top of an EOS body cap that'll theoretically mount onto any EF mount, so you have to be careful, because I get this feeling that bits n pieces will stick out and may damage your camera (again you do all of this at your own risk and don't blame me for your own decisions). DO NOT mount it straight on the camera.
Get your cheap extension tube, and find one that allows the cap and lens contraption to mount with sufficient clearance. Mate them, then attach the completed assembly to the camera. Again you can hot-glue this setup permanently into place, after finding the shortest ext tube that fits.
If you are feeling particularly cheap or ingenious, you can find a way to McGyver the FD lens onto one of the ext tubes permanently, bypassing the body cap. I think it's possible, it's going to be a female-FD-to-female-EOS mount. I'm also thinking "What's the strongest metal epoxy that my hardware shop carries?"
OK FINE, add a couple of dollars more for glue!! 
Off to trawl for a cheap FD today.....a macro for all of $45 plus elbow grease appeals to me.