Hi Farhan, there is no average price for a nib. Nibs can range from $5-$500 depending on type and rarity. If you can let me know what pen you need it for, I will be able to give you better guidance and possibly the least expensive source for them.
We had a slight disagreement some months ago about nib prices. The outcome was that the least you could expect to pay was the "melt price," or value of the gold in the nib. For small nibs, right now the value would be roughly $18--20; higher for bigger nibs. However, my experience is that big (e.g., #5 or larger) command premiums way over the melt price. Also, the older, and rarer nibs ( for example, an old large Waterman's that is flexible) command a higher price. So, check out the rarity of the pen and its nib, the size of the nib, and any unusual aspects of it. Good luck. Dave