Sub-$400.
The SE was released as a phone for price sensitive customers, especially those who haven't bought their first smartphone (i.e. India, select areas of SE Asia). Apple now manufacturers iPhone SE in India just to supply Indian customers. Raising the price above $400 is not a good idea.
What do consumers want? It's clear consumers want larger phones. The top selling phones in India are the Samsung Galaxy J2 (4.7") and Xiaomi Redmi Note 4 (5.5"). The only feature SE2 needs is a larger display.