Without a doubt, the most important thing to consider when buying trading cards is this: What is your reason for buying trading cards? The two main reasons people buy trading cards are to sell them and to collect them, but even within those motives, there are many options and variables to consider.
If you are buying trading cards with the idea of selling them, there are a few different ways to go. You can seek out cards of individual players whom you believe to be underrated and underpriced, then sell them when the players become more popular. The most valuable trading cards are rookie cards, autographed cards and memorabilia cards, which include a piece of jersey or some other material within the card. Cards that are serially numbered - the lower the better - also carry a premium.
You can also buy packs, boxes or even cases of trading cards, hoping to find cards valuable enough to make the investment worthwhile. Buying cases and then selling the boxes within them can be profitable, especially if you are a store buying cases for wholesale prices, but the initial investment is very high. Buying old trading cards can also be profitable, as can buying old packs or boxes. Buying old packs and boxes brings uncertainty into the equation, because you don't know what cards you're going to get, but since the cards are still protected, there's a good chance they will still be in close to mint condition. Cards can be graded, and old cards with high grades will always sell for a premium because they are not easy to find.
Those who simply wish to collect trading cards have a number of options, as well. And any budget can be accommodated; you can buy a 36-pack box of cards from the late 1980s and early 1990s - see 1989 Topps football - and get more than 500 cards for as little as 10 US dollars (USD), or you can buy an ultra high-end "box" that includes just six cards - see any Upper Deck Exquisite release - for anywhere from 500 USD to thousands. The difference is that the 1989 Topps football box will not include any trading cards valued over 2 USD, while every card in Upper Deck Exquisite release is some sort of low-numbered rookie, jersey, autograph or other card which carries a high value.
A lot of collectors have favorite players or teams they like to collect, and like anything else, this can be done to fit any budget. A regular card of your favorite player can cost less than 1 USD, while a numbered autographed rookie card can cost hundreds. You can also collect complete sets of certain types of trading cards, and this, too, can be as cheap or as expensive as you want it to be.
There are also options for where you buy your cards, especially if you use the Internet. Lower prices - and certainly a better selection - can often be found on auction web sites rather than in card stores. There are also price guides available for every sport, so there's no excuse for making an uninformed purchase.