Markup for architecturally significant house?

[Peter Martinson]

My wife's grandparents hired Frank Lloyd Wright to design their home, which was built in the early 1950s.  It's a 4br/2.5ba 1700sqft Usonian style home with a great view (sorry, can't give away the location just yet!).  However, it needs a good $100-150k in repairs/updates, which may be significantly higher due to specialty construction.

Since the grandparents passed away, the children are thinking about selling.  Is it possible to ballpark the selling price of something like this?

Zillow's Zestimate says ~$350k.  A national auction house estimates $700k, and a local agent estimates $600k.  The nutball sister estimates $2 million.  It would be listed on one of the FLW sites, like https://franklloydwrightsites.com.  So, it's likely to be seen by some fans of Wright's work.

Is that $2m a realistic estimate for something like this, or is that just wishful thinking?

Thank you!