what was norman rockwells most valuable piece of art?

There is no way to answer that because the value of art changes and isn't constantly appraised (there may be someone in the future willing to pay a lot more for a certain rockwell in auction than expected). But in general the value of rockwells have increased significantly.

Large paintings of home settings can command $500k and sometimes in the millions.

The record I believe for the highest price paid for a Rockwell (which may be the answer you are looking for) is "Breaking Home Ties" sold in 2006 for $15.4M

With the intention to finally exchange in time. The reason they are regarded illustrators is due to the fact most of the work they did was once commissioned for used as guide cover art or as commercial artwork. Norman Rockwell for example grew to become noted as an illustrator for the publish magazine, some thing he's going to continually be associated with, however is certainly a painter and a excellent artist. Frank Frazetta's art is regarded myth art and he may have a harder time shaking off the ''illustrator'' thing from his resume, but i've under no circumstances heard him complain about it.