Polkadot is like any other cryptocurrency traded on so called exchanges, which determine the price through supply and demand. Polkadot's price is both highly volatile, and somewhat correlated with broader trends in the crypto markets.
We should note also that DOT is a relatively young currency, which points to extra volatility as technical challenges arise and are overcome (or not
overcome).
There are hundreds of projects using the Polkadot network, whose popularity (or lack thereof) will influence the perceived longevity, and therefore the price of DOT.