http://www.nikonchat.com/nikon-d90-kit-prices/

35 mm 1.8 or SB-600 for a D90?
I receive a cash bonus and asked if I either a 35 mm lens 1.8, or SB-600. I want one or the other, unless there is a suggestion that someone can offer me. I am in the price range of $ 300. What I have: Nikon D90 18-105mm Lens 50mm Kit 1.8 Thanks advance
Well, you already have a 50mm f/1.8 for use as a fast prime, so for low light shooting a SB-600 will probably be more useful.
Nikon D90 12-Megapixel Digital SLR Camera Kit with 18-105mm DX VR Lens – JR.com