There's nothing wrong with buying a used dirt bike for your first dirt bike. Used bikes are much cheaper than new bikes and can be bought for between $500 and $2000. It's better to buy a used dirt bike starting out because you might decide to give up the sport soon after starting. You don't want a brand new dirt bike that you spent a lot of money on collecting dust in your garage.  Remember that if a used bike breaks, you have very little chance of a refund if you bought it on eBay or Craigslist. Spending less can often cost more in the future. If you find 2 of the exact same bike at 2 different prices online and the cheaper one is more beaten up, you should consider buying the more expensive bike. You could spend much more money fixing up the cheaper, battered, bike compared to the more expensive bike in good condition. New bikes will generally have a much longer lifespan than used bikes. If you're only getting started with riding dirt bikes, you might want a new dirt bike so it lasts you a few years. If a new bike breaks, you can bring it back to the store and they can fix it or replace it. New dirt bikes can cost anywhere from $2000 upwards depending on the quality of bike you're looking for. The best bikes on the market cost between $7000 and $8000. Most bikes in the $2000 to $3000 range will be perfect for all types of riders and you shouldn't pay much more than that for your first bike. Dirt bikes come in a variety of types and sizes. Since you're only starting out, a play bike will be ideal as your first dirt bike. Don't be offended by the word "play" here. If you've never owned or rode a dirt bike before, it's a bad idea to start out with one of the fastest bikes on the market. Play bikes are designed for beginners.  Play bikes are high on durability and low on maintenance, which means they are perfect for beginners. You'll be able to focus on learning how to ride and control the bike. If you have some experience riding dirt bikes and don't want to buy a play dirt bike, you can buy another type of bike. Trail bikes are the most common type of dirt bike. These bikes are designed for riding on rough terrain. Motocross bikes are built for speed, endurance dirt bikes are made to travel long distances, and street bikes can be driven on roads or on rough terrain. Height is an important aspect when choosing your dirt bike. If you're too big for the bike, you'll be cramped when riding it and more likely to have an accident. If you're too small, you won't have as much control, and you will be more likely to have an accident.  Generally, the dirt bike seat should be half your height. If you're 6 feet (1.8 m), the seat should be 3 feet (0.91 m) tall. When you sit on the bike, you should only be able to touch the floor with your toes and the balls of your feet. You should not be able to plant your heel. If you sit on a bike and can only touch the floor with your toes and the balls of your feet, the bike is the right size.

Summary:
Buy a used dirt bike if you want to save money. Purchase a new dirt bike if you want something that will last a long time. Look for play dirt bikes since they're designed for beginners. Take your height into account when choosing a dirt bike.