There are a number of ways to go about it. No one way is "best". The best value is taking everything into account. Probably the best value amount would be researching past and present ebay auctions and craigslist listings to see what actual deals have taken place. There is a Blue Book to check:
https://store.bluebookinc.com/Home/Default.aspx Gruhn's publishes one too. If you go to amazon under gruhn's they will list about 4 other related publications. Good luck, the research is half the fun!