Hi Helo,
They are pretty expensive when one buys the specially prepared seeds. I believe it has something to do with no fungicides and being sterile. If there are contaminants on the seed husks it is more likely to transfer to the shoot that will be eaten. There is not enough time passing for the contaminants to breakdown in the soil like a normal cycle of seed to full grown brocolli.

I have just purchased 5 different bags of seeds, all in 5lb bags which is saving me a ton of $. I soak the seeds in water and hydrogen peroxide for 6 to 8 hours before I out them in the trays.