Julie - the advice above on checking selling prices on the auction sites like gunbroker or gunsinternational is the best way to determine what they actually sell for, but try your best to find equal condition. this takes time. Many are advertised with glowing praise that are not original or or have had "work" done on them. And many go unsold because they start too high. You also cannot search completed auctions on gunbroker, so you need to find a couple that are similar, bookmark them and check after they close.
for example - here is one from a well known dealer, see if it sells at that price
http://www.gunbroker.com/Auction/Vie...Item=380198044
i see another at about double that that is not original.
this one has had a pad added
http://www.gunsinternational.com/Par...n_id=100368141
this one has been "touched up" incorrectly - but the sale is pending at what I would think is a deal price wise
http://www.gunsinternational.com/PAR...n_id=100395176
Unfortunately, very few of us qualify as appraisers, although very few of us are lacking in opinions either. For appraisals, the average gun shop is even worse.
I am one who thinks keeping family history in the family is a great thing. I would add that the case (although non-Parker in origin) is a great extra. in it current condition the value is nill it may well be restorable to keep with the gun. we have a member - Kevin McCormack - who does this sort of work.