Just like a donation to Goodwill, they give you a form wherein you, the donor, determine the value of the donation. Most people tend to value an unregistered MG at its parts kit value. As far as your comments about proper maintenance and conservation, the vast majority of legitimate and accredited museums/curators take great pains to maintain their collections, even the things in storage.
BTW, for the OP, I spoke with Jesse Smith at length yesterday afternoon. He called to share about a donation of 116 Inf. Regt. (US) documents, a donation of three allegedly unregistered MGs (one of which is registered - he is in the process of helping the owner get it registered to the possessor), ask about a new local-to-me “museum”, and plan for the 2026 Show of Shows. He is still very much employed at the VA War Memorial Foundation.