There are a number of considerations that go into evaluating the value of a tree but for a purely ornamental species such as a Japanese maple, it is worth only what someone is willing to pay for it :-)

There have been various discussions on this forum addressing similar questions, so you might want to review a few to see what was discussed but most boil down to what an eager and willing buyer is willing to pay. For a well-established tree, even a smaller one like 'Garnet', digging and moving such a tree without damage poses its own set of problems and expenses (not inconsiderable) that would necessarily factor into the value.

How big is the tree. Any chance of a picture?