I didn't 'Bing' or 'Google" -search for items like yours , trap121 , but I'd guess you'd get a quick idea from Ebay & such .... @ least you'll get to see what sim. items are being 'offered' for .
Nice vintage sextant , IMO , esp. with original case !
My last comment would be that it sure looks like your item could use a real good cleaning , oiling & checking tightness of all hardware ( will be worth more & show that 'regular' maintenace was performed on the sextant) .
Not unusual to see corrosion on such water-borne instruments , but it is undesirable (except to a buyer who could negotiate on that point) .