USED is the way to buy a macro lens!!! They get bought, used for a month, shelved, then sold off two years later. For the most part.
Three off-brand autofocusing lenses stand out to me:Tamron 90mm AF is good. Tokina 100mm f/2.8 AT-X is good. Sigma 105mm f/2.8 EX macro is good.
In manually focusing lenses, the older Nikon 105mm f/4 and f/2.8 manual focus Micro~Nikkor models are all quite good. The D300 can light meter with the older Ai- and Ai-S moiunt manaul focusing macro lenses, so there are a LOT of good, used choices for a D300 shooter. Vivitar actually made some VERY nice macro lenses 30 years ago, that were built like the proverbial brick house,and have good optics. Same thing with the older Tamron Adaptall macro and the older Tokina macro lenses. Five or six years ago, Phoenix and Samyang made a 100mm macro lens that was surprisingly decent, and it sold for $109 brand new...