Tamron 18-200 for Nikon D3100

The Tamron AF 18-200mm F/3,5-6,3 XR Di II LD Aspherical [IF] MACRO (model A14) with built-in motor (can be AF on your D3100) is cheaper than the Nikon AF-S 18-200mm DX, The Tamron lens (model A14N : Non built-in motor type for Nikon) was discontinued from 2008.

However, you can buy it as new or second-hand on eBay or B&H... or Amazon, here is the price of this Tamron on Amazon as new ($299,00 USD) or used ($217,00 USD):

http://www.amazon.com/Tamron-18-200mm-3-5-6-3-Aspherical-Digital/dp/B0015U95SW/ref=sr_1_2?ie=UTF8&qid=1358884194&sr=8-2&keywords=Tamron+AF+18-200mm+F%2F3%2C5-6%2C3+XR+Di+II+LD

Francois.