How do you value subsets?

[JasonL]

How would you place a value on a subset or a lot of prewar cards?
I just began a subscription to VCP, where you can look up cards individually, but to my knowlegde you can't look up previously closed sales of card lots.

any suggestions?
do you just sit and flip through old auction catalogs to find comparisons?

Here's an example:
If I wanted to sell my T206 Cubs team set (which I don't, and this doesn't belong on the BST ), how would I figure an asking price? or a value to reference as one of my holdings/assets of the larger collection?

Thanks in advance for your thoughts...