~$400
Your request is very vague. If you need new, check Murrays.com, as they have the most complete listing of Nacra parts prices on the internet (i.e, easily available). I know they have dagger boards listed.
Please don't freak at the price. If you can afford a I20/N20, you will break things and need to replace them; I recently spent $1000, with team discounts, on replacing my standing rigging and re-fitting most of my running rigging (cleats etc.). I will get some of this back out of the boat when I go to sell it, but that is likely a few years down the road.