Good wide-angle lens for Nikon D3100?

I'm looking for a good wide-angle lens for my Nikon D3100 as I enjoy taking photographs of landscapes. However, I'm a student so I don't have much money and won't be able to afford a lens like the Nikkor 10-24mm lens. I was wondering if anyone knows of any decent lenses at a reasonable price.

Added (1). My Nikon D3100 came with the 18-55mm lens… Would this be good enough for landscapes as I'm only a beginner or should I still think about getting another lens?

Yup - I use the Sigma 10-20mm lens on my D300 - great little lens, decent quality & good value for money.

Edit - the 18mm will be fine for landscapes - but if you want more dramatically wide landscapes than that gives you, then you'd need something in the sub-18mm range.

See how you get on at 18mm or thereabouts.