I paid $60 for an '83 600, (same frame) but it had been rattlecanned blue, including the headset.
And FYI: the 531c is main triangle only. The forks and stays are Tange Mangalloy 2001.
Depending on the real condition and the seller motivation you could look anywhere from $200 to $400. I wouldn't let mine go for $400, but it's not exactly OE anymore.
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72 Frejus (for sale), Holdsworth Record (for sale), special CNC & Gitane Interclub / 74 Italvega NR (for sale) / c80 French / 82 Raleigh Intl MkII f&f (for sale)/ 83 Trek 620 (for sale)/ 84 Bruce Gordon Chinook (for sale)/ 85 Ron Cooper / 87 Centurion IM MV (for sale) / 03 Casati Dardo / 08 BF IRO / 09 Dogma FPX / 09 Giant TCX0 / 10 Vassago Fisticuff