Hancock seed Co has 5# bags of Dakion {Trophy} Radish seed for 18.99 with free shipping, that's cheaper than any where else I could find. some sites want 12 bucks a pound for the same seed.
I've been told that Nitro and Daikon are essentially the same radish sold by different companies, if so you can get the Nitro for as low as $1.65 a pound. I almost ordered a bag this year to mix in but went a different direction.