If the dollar amount is around $100.00 just wait until you have the difference. Having the grip will not make you a better photographer or make your pictures better. It's not worth the risk of losing the warranty on your camera. To put it another way. Who among us would not purchase a warranty for the D300 if it only costs $100.00.
Our society has become fixated on getting it now no matter the risk or cost. Learn the enjoyment of saving up for something then purchasing it. There is a lot of joy in saving for an item and not going into debt or having to settle for something less. I have done this with each of my purchases of lenses and the D300 itself. I refuse to go into debt over having something right now vs saving up for it.