At US$400, your options are pretty limited.

• The Nikon 40mm macro lens is the cheapest option (about $275) and from all reports it's a solid lens. It'll autofocus and meter with the D7000 and any other modern Nikon body. But... you have to get thisclose to your subject to use it. So close that you are practically touching the subject. The minimum focus distance is 6.4" from the sensor plane so that's about two inches from the front of the lens to take a photo of something at a 1:1 aspect ratio. There's literally no room to light your subject.

• The Tokina 100mm macro lens is right at your budget ($400). It's an older lens but an advantage of the D7000 is that it will let you use autofocus with this kind of lens. Working distance will be good and results will be pretty good. All around, a solid buy.

• The Nikon 55mm macro lens is right at your budget ($400). A very old lens, your wife will need to manually focus and meter with the lens. It only focuses down to half life size, a 1:2 ratio, but that's often still plenty to take photos of things like flowers. A great lens, if you know what you're getting in to.

Push the budget up to $500 and several really good options become available. Nikon's older 60mm, Nikon's 85mm, Tamron's 90mm, and Sigma's 70mm.

Another direction, as rob describes, is some extension tubes. You won't get near as close as with a true macro lens, but maybe that's not necessary. Reversing rings might also work well, if you've got a slew of lenses to work with.