Clarinet reeds vary in price based on the clarinet type they are used for, the brand that produces them and the strength they have. A box of 10 is usually between $20 and $30 and it should last you a good while. You can find bigger packages, and the more they are in a box, the better the price for each of them.
Brands that are not too well known sell them for around $10 a box, but that doesn’t mean they are not good. On the opposite side, reeds specially made for a certain type of clarinet and targeted to more advanced players can reach $40 per box.