There is the other element of so called "collectability". A lot of people that buy ML'ers buy otherwise collectible guns too, and apply those same buying standards. If a gun is at 70% finish, if it's a factory finish that's different than cousin Herb's original finish, even though both might be identical in present state. The function and utility value of each will be the same, but the secondary or tertiary markets for re-sale or re-re-sale are thinner for a kit gun than they are for a factory gun, which is to and of known quality standards, however low or high they may be for that particular factory.
However, if this home built gun went on to become the prototype for a model that then became a historically significant piece, such as John C. Garand's prototype for the M1 Garand rifle, even his pre-production run of hand-built guns from the Model Shop are of VERY significant value (in the 100's of thousands of dollars) because of their place in shaping world history.
If the individual gun performed some significant world event, like the gun that shot someone of significance, then that too would affect the value.
I haven't a clue what his actual M1 Garand prototype hand built by him would go for, but I'm sure it would be more than $1,000,000.