The cost of a domain depends on whether it is available, already owned or classified as premium by a registry or seller.
An available domain usually has a low yearly registration fee. The exact price depends on the extension, registrar, promotions and renewal cost. Always check the renewal price, not only the first-year discount.
A domain already owned by someone else can cost much more. Short names, generic terms, strong keywords, aged domains and domains with traffic or authority can sell for hundreds, thousands or even much higher amounts.
Some registries also reserve premium domains and sell them at higher registration or renewal prices. In these cases the domain may technically be available, but not at standard cost.
When evaluating a purchase, consider both the acquisition price and the long-term value for the project: branding, trust, advertising efficiency and resale potential.