Originally posted by Ash You'd be looking for a second hand manual focus lens like the Pentax A 100 f/4 macro.
It's just a matter of finding one now - check eBay if the forum marketplace is not offering one in your desired timeframe.
i doubt you can get a 100mm macro at 100... macro lens are expensive regardless of manual or auto focus, because the fact of the matter is, people shoot macro will be 99% of the time manual focusing, making auto focus rather useless (but it's still is a bit more expensive as you can use the lens for regular shooting).
As expected, i checked with KEH (cheapest source around that I've found), 150 ish for a decent condition one...
I would suggest a 50mm macro for 100 dollars. you ain't shooting any bugs because of the working distance, but flowers and non moving subjects shouldn't be an issue.
extension tubes, reverse lens... they'll work, but i'd still suggest a macro lens, it makes your life a whole lot easier, those cheap method will always have a fault... stick with a macro lens and save yourself the trouble, even a cheap ass like me had to shell out the money for a macro lens.