When you say what they're "worth", online sales (ebay, etc) are by-far your best (possibly, more accurate), but to have a set price for something like insurance, etc, then a "guide" like Beckett is ideal.
An online subscription (say 1 month), will get you access to practically EVERY card in that guide (Basketball, or Football, or Hockey, etc), much the same as the actual magazine. Just the main difference, is the online one will expire, whereas the magazine will remain in your hands for a lot longer, but as I said man, the down-side there, is the magazine won't contain all the cards you want to source/check.
Online subs are also great for obtaining checklists, etc. They do take some time getting them down, but they're still worth while.