Nightshade has a point about buying secondhand lenses.
Your 70 - 300 is going to be difficult to focus at F11 (at the Long end) with a 2x attached. Plus you are going to need some good support (tripod) to keep a 600mm lens at slow speeds from blurring.
I have seen some good bargains on
Digital Camera Reviews and Digital Photography Techniques in the classified section in fact there are two 135-400mm lens for sale as I write, both nikon fit.
For macro work (depending on what you want to do) try getting a extension tube. They can be used with converters and are reletively cheap. Ext tube on macro lens works but you have to be close - flowers and still subjects. Ext tube on a 300mm would be great for insects though.
Jason