What is the highest zoom lens for Nikon D3100?

Any good telephoto lenses for under $500?

There are some available for that budget but they're not the highest quality. Here's a good place to do research on Nikon lenses.
http://www.bhphotovideo.com/...video.com/

There are two with the longest zoom range, the AF-S 18-200 mm and 28-300 mm. Neither are $500

The AF-S 55-300 mm is your next best choice, under $400 Added to your 18-55 mm lens you will have a good reach.

The Nikkor lens with the longest reach is the AF-S 200-400 mm VR, again, not in your budget range

I think you mean what is the longest lens for Nikon under $500. Zoom and telephoto are diffrent things.

The longest zoom range lens I can think of is the 50-500 Sigma, $1200.

The most telephoto lens that you can get at the store is 600mm, $7000.

If you want to go long and cheap, the 70-300 goes for $370.