When looking for prices the first place to start are the authorities. There are three main companies:

Professional Sports Authenticator (PSA)
PSA is the world's largest and oldest grading service. PSA grades are generally considered authoritative anywhere online or offline, and since it's a well-established company older grades still carry their weight now just as they did when fresh, so your investment in a professional grading from PSA will still make sense ten years from now, or twenty or more. PSA also grades cards and memorabilia from all other major sports.

Beckett Baseball
Beckett is arguably the most commonly cited price guide available. While imperfect -- it's hard to keep up with a dynamic market using a print magazine these days -- Beckett is still the best choice of what to have in your back pocket when hitting the shops and shows to buy or sell baseball cards. Pricing baseball cards is not an exact science, so bear in mind that the Beckett guide should be treated as the rough guideline that it is. But if you've never seen it, it's certainly worth grabbing a copy or even subscribing.

GMA Grading
GMA is probably best known as the company that sells tons of cards and collectibles on late-night TV ads, home shopping channels etc. This is not to diminish the quality of their work; in fact on more common issues a GMA grading makes sense due to their much lower prices. GMA's own approach to grading is with the reseller in mind, not just those collectors interested in preserving their cards, and their extensive experience selling on television and the Internet pays off in this approach.

What if you don't want to pay for grading and pricing services? The best best when pricing baseball cards on a budget is to check the master of all marketplaces: eBay. Every day literally tens of thousands of cards are listed on eBay, and thousands bid on by buyers from kids spending their allowances to professional investors. It is an all-access movers and shakers market, and you can get a wealth of pricing information from it.

Pricing baseball cards is not an exact science. As with any other item that fluctuates in value, the true value of your card at any given moment in time is whatever the top dollar you can get from a buyer that day is. However, online and offline price guides and the constantly-updating data in the eBay marketplace can give you ideas of the range in which a given card or series of cards tends to sell, and this information can help you make a decision on whether those cards need to head down to the market now, or sit in your collection to mature a little more.