I do not remember paying anything for the e-tag, but perhaps a deposit.
If there is no significant value on the tag, it might be easier to just get a new one. The battery lasts years, and I've never had one go bad, but I've gotten new ones when I got new cars and when I had the film replaced on the windshield just for the holder.
Incidentally, I think the easiest way to top-up is using your e-banking app, no need to go anywhere.