In my opinion, its a great idea to look at "sold listings" on ebay to start getting an idea of what you're looking for costs, generally. keep in mind that later, if you find the same(ish) card NOT on ebay, you can usually count on paying a little less.
I think that this is one of those things that you just get a feel for as you browse prices over time. Ebay is the largest source of research material that I know of.
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