The price of a clarinet varies, it depends on where you choose to buy from. Most clarinet are expensive based on the the quality of materials used in production. The quality of wood work, mouth piece and keys determine the quality of sound produced. The reed has to be changed from time to time in order to maintain the quality of sound produced.
This means buying a clarinet is not a one time purchase, maintenance cost is also some thing to consider. A clarinet with good production mechanics will produce a good sound than a clarinet with not too good materials or poor mechanics put in the production. If you find the new clarinet expensive, you can buy a used one which will come at a cheaper price.
The term 'expensive' is quite relative. If you start off with a basic clarinet like a Yamaha, it won't cost a whole lot—especially if you buy used. Working your way up into more professional clarinets, they do tend to get more expensive. This is because of the workmanship that goes into them, creating the keys and the perfect mouthpiece which will create a beautiful, quality sound.
However, no matter how much you spend buying a clarinet, what will matter is the amount of practice involved. No one will be able to pick up a $10,000 clarinet and play it well if they don't learn and practice.