Who owns the impressions an orthodontist takes of your teeth?

My daughter lost her retainers. It is my understanding that the impressions taken by the ortho are sent to a lab with an RX to make the retainers. If I have already paid for all my orthodontic care why would I be responsible to pay for the mark up on the new set of retainers? I have found pricing that shows the orthodontist marks up retainers 200% on replacement. This may be incorrect, but still, if I have already paid for the impression and molding shouldn't I be able to get the replacement at cost?Or, perhaps with a reasonable processing fee? Please help me out!