Airlines only release a certain number of seats at each price range and once the cheapest seats have sold out you move to the next ‘best price’ band. The earlier you book, the less you will pay. If you’re really organised and book several months in advance you may find that you have paid a lot less than the person sitting next to you.
Airlines do reserve the right to sell seats at varying prices depending upon current volumes of availability, and demand.
Taxes change constantly, and can vary from minute to minute. You will be advised the exact tax and fees when you make payment.