Cheap lens for Nikon D3100?

Hi I'd like to ask for a suggestion regarding my d3100's lens. What would be the best and cheap lens i could get? My style varies, sometimes i'm into micro (which the kit lens can't do), sometimes i'm into scenery and sometimes i do portraits… So i don't know which lens i should get where i could use it in general. I want to upgrade my lens which captures more precise images where subject really stands out.

50mm f1.8 is usually a cheap ish option, and a good intro to prime lenses (no zoom)

Not cheap but the least expensive option for your D3100. Look at the Nikkor AF-S 50 mm f/1.8 lens (about $220) and an extension tube set with electrical contacts, about $100. With those extension tubes, you will be able to use the 50 mm lens as a macro lens, not a cool maybe as the 105 mm f/2.8 macro ($985), but certainly within your budget. The 50 mm f/1.8 is also an excellent portrait lens.

I hope you know that to shoot landscapes (scenery) you will need a nice wide angle zoom like the 190-24 mm zoom lens ($900). The lens you have, the 18-55 mm will do landscapes, but they will not be as brilliant as the 10-24 mm lens.

The only way for you to capture more precise images, is for you to learn how to handle the camera. Buying a new lens is just throwing money at a problem that has more to do with your skills as a photographer, than the equipment you have. Cameras and lenses are just tools and Nikon makes some of the best tools for photographers on the planet. Just because you are using an entry level camera and entry level lens, does NOT mean you can't produce amazing images. That is all up to you honing the skills you have, using the equipment you have

The kit lens can do scenery and portraits. Get a micro lens for macro shots. After that, your next lens would probably be a 50mm f/1.8 for better portraits. Then if you really want to get wider scenic shots, invest on a hefty wide angle lens, wider than 18mm.

Sigma makes (or used to make) a macro lens 70mm f2.8 which had very good reviews and was not hugely expensive. It would be good for portraits and close-ups. A bit too short for me for landscapes, but I'm unusual in that I prefer to select bits of a landscape with a tele lens.

The Micro Nikkor AF-S DX 40mm is the least expensive macro lens, and is reputed to be very sharp. Its focal length is shorter than ideal for portraits. The working distance is short for some subjects like insects. It might be good for flowers if you can position the camera right. A good tripod should be high on your list. Whatever lens you get, you won't make best use of it without a tripod.
The MIcro Nikkor AF-S 60mm would be an excellent macro that could easily double for portraits, however, it is getting up in price.
If you had a D7000 the Sigma 70mm macro could be a fine choice, but it won't autofocus with your D3100. If you read about the joys of manual focusing, before buying anything switch off the auto focus on your kit lens and see if you are happy with that.

Cheap lenses give you poor photos. Save your money for a good lens.

Well for micro it's a special lens. For most other things, you can get one and do a lot of stuff. What is your max price range? 35mm and 50mm f/1.8 are great for portraits and everyday stuff, also in low light, 55-200mm and 70-300mm are telephoto's on a budget, and 16-85mm and 18-105mm are all around great kits.

Here's a Lens Buying guide - http://www.the-dslr-photographer.com/2009/11/buying-a-lens/