It really depends on what range you are looking for and your definition of affordable. The most common choices in Nikon are the 55-300vr, as low as $250-$300 sometimes, or the 70-300vr at about $550 (I have this one, great lens).
I also had the Nikon 55-200vr, which is very inexpensive, but you will eventually want one of the above.
Whatever you choose stick with one that has VR, even if you go to a third party lens. There are allot of cheap non-VR lenses, but you will need to use a tripod most of the time.
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