i got my bite for free, and then gave out bites for free as well. Vampire solidarity! (that was during the days when you could see "50k vampire bite" in the chat)
Maybe there is a limitation of purity. Like the ones that got it from "infection" instead of genetics are not pure, so they cant infect more themselves.
both vamp bites and werewolf bites are free now in eso, even in zone chat. i remember when people used to charge for it in zone chat but the supply is so high that it was easy to ask a friend or guildie to give you a free one.