One of the problems that we collectors run into when we want to get an idea of a "value" of our cards is the lack of guides out there to help us. True, we have Beckett, but anybody who's been around the hobby long enough knows the issues with them. Beckett has Topps and Upper Deck in their pockets!
I mean little things such as "Hey, we'll give you guys the exclusive on this upcoming product!" or "We'll give you yet ANOTHER loaded box that the regular collector never gets!" in order that Beckett price certain cards, well, a little higher than they should be. Is this conspiracy? No, it actually happens.
Anyway, I would like to open your eyes to a couple of other options out there for you other than checking eBay every day for a completed auction. Which, by the way, is probably as close to a true value as you're going to get, outside factors not withstanding.
Sports Collectors Digest Standard Catalog of Baseball Cards. This book is issued once right near the end of the year. The one problem is this guide only comes out once a year so you are not going to find values on your recent cards until the end of the year. What this guide does offer you is a listing of every card from pretty much every set out there. There are cards in here you didn't even know existed. On top of the listing is a photograph of a card from the set along with a brief description of the set. Not only is this guide a great reference for cards, the prices are also much more realistic in regards to "value". When I say more realistic, I mean the values assigned are much closer to realized buy/sell prices than Beckett it. Recently, someone had emailed me asking about the price of a set that Beckett had listed as N/A. This guide had the set price and I gave it to him. It was right around what he expected. Another person emailed me asking about a card price. According to Beckett, the card had a value of $30. This guide had the value listed at exactly 1/2 of that at $15. Two recent auctions of the card ended at $11.53 and $13.54 respectively. I'm just sayin'.
I would also like to point you in the direction of a great web site service for you graded collectors out there of vintage cards. The site is VintageCardPrices.com. What this site does is keep track, via ebay, of completed vintage baseball cards that have been graded by SGC, PSA, BVG, and GAI. They keep track of each ending price by grading company and grade itself. The list of cards they track are from 1867-1959 and I have read that they intend of making that list longer to reach into the newer market from 1960 on, though I'm unsure of how new they will cover. This is a paid service but if you are a heavy vintage collector, this may be worth it for you. In my opinion, it's a great service to show realized prices of cards that you might be trying to obtain yourself and you can avoid getting ripped off. There is so much more to this site that I could ramble on forever. Just check it out!
Anyway, I hope this was helpful in some way, shape, or form. I know many of us feel that there is only one option out there but really, there is much, much more!