The Tamron 90mm f2.8 SP AF Di has been a great lens for years -
B&H has it for $459.95 I sold mine for Nikon mount a few years ago.
Extension tubes and reversing rings are useful for folks who want to shoot macro on a strict budget, but anyone shooting macro very often will quickly tire of the severely limited range of focus with extension tubes or reversing rings... I do use extension tubes with my Tamron 180mm macro fairly often to get at greater than 1:1 life size.
Sigma makes a fine 105mm macro, and a 50mm f2.8 EX DG Macro for $369 at B&H.
Nikon makes a lovely 60mm AF-S Micro Nikkor f2.8G ED macro autofocus lens for about $550.