Gas is priced by network demand. On Ethereum a simple transfer might cost a dollar or fifty dollars depending on congestion; complex defi transactions cost more. Layer-2 networks and other chains charge cents.
For active dex traders, gas is a real cost that can exceed the bid-ask-spread equivalent on small trades.
Example: during a busy period a token swap on Ethereum mainnet costs $35 in gas. On a $500 trade that is a 7% fee before slippage.