Those Kiwaya KPS models with the wide neck and beautiful construction quality are really great, but I don't know anyone outside of Japan selling them for less than US$600.The vintage Martins are fantastic, but they have narrow necks, not the wider ones the OP is hoping to find.
The Kiwaya models with the wide neck (38mm) can be found at the link below. I think there are Famous equivalents, but I don't know where to find them.
KIWAYA
www.kiwayasbest.com
The link below is for a Mahalo Soprano with a 40mm nut width. It isn't much of a ukulele (costs about $50), but the wide nut is a wonderful experience if you like a wide nut width.
Kahiko PLUS Series - Soprano - Mahalo Ukuleles
Mahalo Kahiko PLUS Series - Soprano. For players seeking a step-up in features. Equipped with Aquila Super Nylgut strings and Graph Tech NuBone XB saddle.mahaloukuleles.com
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