Macro lenses tend to be on the pricey side.  If you are willing to
play with making your own end caps and manual everything, you can get
a reversing ring and any 52 mm filter lens (you can use your 50 mm).
It works but it's quite clumsy.  Also, you can buy extension tubes.

Used macros show up from time to time at KEH and other places.  The
60mm AF is one of Nikon's sharpest lenses.  New, it's around
$350-$400, so not cheap, but not really expensive, either.

I found a sort of rig up which works for Nikon in my spares - a manual
BPM bellows unit with a Nikon mount, and a Leica thread suitable for
enlarging lenses at the other end. My Rodenstock Apo 75mm f4 gave
excellent sharpness but a big flare patch on the D200 from my typical
perspex-table illumination, sensor reflection I guess.

If you can find one of either, I have owned two Nikkor lenses which were
great on bellows. One was the 120mm f5.6(or 6.3?) HX Nikkor intended for
some kind of opthalmic equipment, probably removed from a special camera
- a camera store junk bin discovery. Another was an El-Nikkor which I am
sure was a very peculiar focal length - 63mm f3.5 or something. It was a
special lens for use copying slides and recommended for the old Bowens
Illumitran when copying both 35mm and rollfilm. I got one for this
purpose probably 30 years ago, and after I had stopped using it for
that, discovered it was a very good macro lens.

Though enlarging Nikkors sometimes sell for high prices, it's equally
possible to find them - and manual bellows units - very cheaply.

I've done some really nice creative full aperture macro work with a 55mm
f1.8 Super Takumar (Pentax thread) mounted on bellows the same way using
a 49mm to Leica thread 'reversing ring'. The Nikon 50mm f1.8 would need
a 52mm reversing ring and work well that way round.

If you have a set of close-up 'filters' (they aren't really filters,
they are supplementary lenses), a bellows unit can also offer another
unique surprise - if you have enough c/u lenses, or a powerful enough
like those 10 dioptre models offered as a novelty c/u power, you can
tgape them directly to the opening of the bellows front and use them as
an imaging lens in their own right. Beautiful soft focus, dreamy fx and
no control of aperture but you can cut shaped apertures like slits or
stars.

Digital SLR bodies make all this so easy to experiment with, not like
film when it took ages to check the results. I'd rate a set of manual
bellows and a box of old lenses or even old lens BITS as a real asset in
a DSLR kit.

David

Tom,
Try to find a 2nd hand 55 or 60mm Nikon Micro, just bought a 55 f2.8 AF
for $200. If you want to photograph bugs etc (small creapy things) a 90
Tamron is a good buy (also can be found used). With the D50 you want an
AF lens so macros won't get to the 50 f1.8 price range. But if you want
to do macro a true macro/micro (Nikon talk but true) lens does wonders.

Tom