The Tamron 90mm macro at B&H and probably many other places is selling for $449 and there is a mail in rebate that ends April 30 giving it a total cost of $399. I've applied for Tamron rebates on two separate lenses and Tamron has been very prompt. The ones for Nikon are designated BIM which means built in motor for those times you want to use auto focus if you have a camera body without a focusing motor. The warranty on Tamron lenses is six years and I register mine via the net. It is not internal focusing but it is a good sharp lens.
Buy used... if you are looking for cheap! You can usually find a Tamron 90 or a Tokina 100 for $300-$350 (Amazon has both at $355 used right now... Ebay might be cheaper.. so would Craigslist)
I believe I would find the in focus range too limiting when using extension tubes. With a macro lens you can focus to infinity. Most flying insects will not allow you to get within inches of them.
The Cosina 100mm f/3.5 macro (with the diopter that comes with the lens, it is 1:1) if you can still find them (used or new). I saw it once in awhile on ebay for a little over $100.