You can get a reasonable idea by the average price on the likes of gun trader/gun star, but remember an asking price is not always the sale price.
2/3 of what they can be had for new is a guide for something that's in as new condition, but I don't see one tin or ten tins makes any difference, it's still 2nd hand with no warranty.
I'd agree splitting rifle & scope might be a good idea better still offer the option of either.