There seem to be a good many of them out there (especially of the import marked variety), always lots on GB. That price seems about right but I would want it to be in really nice condition for that since there always seem to be lots to choose from. If you're out shopping for one of these always be on the lookout for the Nazi contract guns, as they bring quite a bit more $$ (serial numbers 92851 to 98850).
An original should be rust blued. About 20 years ago a large quantity were imported that had horrible bores and had been dipped blued. If buying the first thing to check is the condition of the barrel. These pistols should only be fired with 9mm Largo ammunition.
If it's marked "9mm (38)" on the slide and "9mm & 38" on the barrel it's safe to shoot .38 ACP in them as well (NOT .38 Super). The back face of the barrel chamber will have a thin groove cut in it for the .38 semi-rimless casing.
An original finish gun will have the interior part of frame and slide "in the white"....
The pics below are from a gun that hasn't been modified for .38 - most that you'll see on GB etc have been "upgraded".
Back to the original question, if it's not original finish (or a factory arsenal rework) I wouldn't mess with it. If it's a nice crispy minty factory-original condition, $375 is a very fair price. Run the price down with condition accordingly. If it's something special (like this Chilean Navy contract) than double the price. To me, if it doesn't have any import markings and hasn't been converted for .38, I give it a bump too (20% or so).
Most all will have "Made in Spain" stamped somewhere on it, that's an "acceptable" import mark. If it has a full marking (especially a nasty-looking one) I'd take a pass as there are so many nice examples of these out there.
If you're looking to shoot 9mm Parabellum/Luger, then find a nice Astra 600 instead.