Everyone is competing for the top of the page. Having high scores for keyword and QS gets you a better rate, but then you need to outbid others who may have the same scores and possibly a better track record. The price can change at any time once you start the campaign, but usually it will only go higher as others seek to outbid you if you reach the top. It is a multidimensional calculation. Books have been written to help people develop a strategy, or experience will teach you - slowly and painfully.