Jumpeating is free, but last in priority.
Also, keep in mind, in general, only pilots can take the actual jump seat. You may be lower priority then other non revs trying to get on but if you are the only one that can sit up front, then you will get on and they won't.
Can you request a jumpseat though, even if there are seats in back? Say you are a newhire on an ERJ and you are flying home or something on an ERJ and you would like to understand the cockpit and flows and how the aircraft systems work and whatnot...could you request to sit in the jumpseat as sort of some "extra" training?