Valuing Vintage Ukes? How can you tell

Can any direct me in the way of trying to figure out what the value of Martins and Kamakas are. To make it simple, I was thinking about the following catergories for each brand:

2 sizes: Sopranos and Concerts
Condition: Excellent, Average, Needs Works
Decade Built:
Approximate Value:

If there is a thread or website, I would love to look at it as I want to buy an older instrument as if there are no issues, the wood should have aged and sound great. Thanks

For a start you can have a look here:

http://ukulelehunt.com/buy-ukulele/categories/ukulele-brands/

More buyers than vintage Martins/Kamakas= high value
More vintage Martins/kamakas than buyers=low value