DaveyDitzer Loc: Western PA

OP stated the camera is a D40.

Mark Sturtevant Loc: Grand Blanc, MI

There are many responses. A true 'budget' option that will still take great pictures is to take a small prime lens, like a 50mm, and increase its close-up capabilities by using extension tubes. These are used in combination to be able to focus closer and closer. Sometimes you won't need extension tubes with the 50mm, as it can focus pretty close on its own.

Use extension tubes that let the lens electronically communicate with the camera so that you have aperture control. It is common to stay on manual focus for this sort of work (since automatic focus is often confused for close ups). Shoot at smaller apertures (f/11 or smaller) to increase depth of focus. Use strong lighting. A soft box and external flashes are often used to make even, diffused lighting for product shots. Consider spending your $ on getting good lighting and diffusion rather than the lens.

ramarsh

Find a used Tokina 100mm f 2.8 lens on eBay.
Vastly underrated.
No stabilization.
You'll be using a tripod anyway.

Leitz Loc: Solms

The aperture, of course, has zero effect on depth of focus.

Bill P

And now we take a short break from discussing meaningless technical subjects, and get practical.

A slightly longer lens, like a 90-100mm will give you needed distance from the subject that will make lighting much easier.

We now return to our regular programming.