Foam core residential entry door, plain slab are around $100.
No door knob holes, no window. Cut to height, drill knob holes, attach with a piano hinge.
Windows and inserts can be cut in anywhere. I would install one with a opening window with screen, and built in blinds. However a insert with all that cost almost as much as the door.
All it takes is a few metal pieces formed up to make the opening.
Good R value for a door
, and quicker than making your own.