I'm one of those recommending the Tamron 90/f2.8. It's normally available for about $275 in mint used condition at KEH.com. You will inevitably read about how terrible non-Nikkor lenses are, and how some folks will never get a non-Nikkor again. Well, here are two shots from the Tamron:
These were both taken quite a while ago and I think the photographer was the limiting factor in both cases.
I also currently own the older 105/f2.8 AFD Micro-Nikkor. It is slightly behind the Tamron, optically, but as you can see from these, "slightly behind" is still pretty darn good. The current 105/f2.8 AFS VR Micro-Nikkor (the one whose price you don't like at around $900+) is also slightly behind the Tamron. Its price is partially justified by the fact that it has nicer focusing, has VR, and does not extend during focusing. As you can see, I continue to use older lenses without those features, and it doesn't seem to get in the way. I've also used the older Sigma 105/f2.8 AFD Macro (the version without HSM and OS), and if anything it may be a bit better than the Tamron in sharpness. I've never used the Tokina 100/f2.8 AFD Macro but it too is about the same price (about $500 new) and it too is in the same league. Unfortunately the current Sigma 105/f2.8 HSM OS is now up to $1000, even more than the Nikkor.
> Looking at a f2.8
Not that I really think that some other lenses would be better, but macro is one area where maximum aperture doesn't mean very much. In macro one is nearly always struggling to get enough DOF, so it is very, very uncommon to be shooting macro wide open, regardless of what that aperture is. An f/3.5 macro lens is pretty much functionally the same as an f/2.8 or even f/2 macro lens, at least if it's being used for macro work. You'll inevitably find that you need to stop down to f/8, f/11, or even more to get enough DOF. The one slower macro lens you might consider is the 85/f3.5 AFS VR DX Micro-Nikkor. All of the above are optically better, and cover FX frames as well as DX. Still, if you do your part, this one won't let you down either. BTW I do practice what I preach here. I also have an f/4 macro lens (the 200/f4 AFD Micro-Nikkor), and really, it is pretty rare to shoot that one wide open.
I would avoid the shorter macros as a first macro lens; they put you right on top of the subject, making it difficult to avoid scaring live subjects (insects). For non-live ones that can't get scared and run/fly away (or worse, attack - bell hornets!) the 55/60mm can be so close as to interfere with lighting the subject. I also have the 60/f2.8 AFS Micro and while it is a superb optic, the 2" working distance can be a problem at times.
_____ Brian... a bicoastal Nikonian and Team Member
My gallery is online. Comments and critique welcomed any time!