Hello!
Clearly defining what functionality you are looking this "add-on" to have (both front end admin side) would help better assess the work and expenses involved in its development and, maybe, get the bids down. With the little information you have provided there's no way of telling what the "proper" big could be, and everybody is "betting the safe bet".
So if you can lay out, in simple words, what you actually want this woul dbe greatly appreciated, by everybody.
Kind Regards,
Dobri