The least expensive option is the nikon 105mm f4 ais manual focus lens. It won't meter with your d3100 but that isn't as big of issue as many people think (dslrs have instant exposure feedback), and macro work is best focused manually anyway.
Typically people use extension tubes when doing macro work--which let you focus even closer. If you want to maintain autofocus and metering with an af-s lens you need "cpu" tubes. Manual tubes, which u use with mf lenses, are MUCH cheaper then computerized tubes. So when u factor in all the costs the older mf setups are much cheaper--around $200, instead of $700.