Dan, flatbolt prices have been soft lately except for the rare calibers. 6mms are the third most common caliber after the .308Win and .243Win. Without sights or box I'd price it in the $650 range. May go a bit higher due to the four-digit serial number if someone doesn't have one in the collection. Prefix serial numbered rifles are selling in the $450-550 range.
Not a lot of flatbolt collectors leads to big variations in selling prices. One rifle will bring a good price and the next one will sit for sale for a long time at the same price.
The rarer calibers, 6.5 RemMag, .284Win, .250Sav etc., are selling better.
JMHO