Not always true.
Of course, when someone builds an instrument out of extremely figured Koa, the eye candy effect will always take over. Its the true players that close their eyes to find their instrument, or they have a custom made.
James Hill plays one of my favorite combinations, and Jake's Spruce top had more complexity that his current `ukulele, imo.
When talking to kupuna about `ukulele, the statement "that old Martin sound" comes up, and is usually referring to Mahogany instruments.
Why it costs so much is a matter of supply and demand, and has been covered above.
Of course, don't get me wrong. My current go to `ukulele is made of Koa, just because the Milo is not up and running, yet.
-Aaron