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.

Try Tamron. They have some of the "cheaper" lenses that have long reach and do macro at the same time.

It will AF on any Nikon body.

For that kind of shot, you might want to get a shorter macro lens to put you much nearer to the subject.

I bought a 8 mm cs mount and a 500 mm T2 mount each for 20 bucks used including shipping. Both work well on my camera with adapters.

Try ebay for used, and Amazon for new.