Just to add on to what Tom said, the pre-made decks that you can buy for the most part won't really be competitive against any custom-built deck by a competent player. However, if all you're looking for is a cheap intro to the game or just want to mess around, they can be fun. I have an out of town friend I only see every couple months, and we are extremely casual players. Every time we get together we'll each grab another deck from the local card shop and add them to our growing pile of decks to choose from, and it's a blast. They're anywhere from 5 dollars on up for a complete deck, so it's hard to go wrong.