what determines the level of market power a monopolist can exercise
Market power A firm usually has market power by virtue of controlling a large portion of the market. In extreme cases—monopoly and monopsony—the firm controls the entire market. However, market size alone is not the only indicator of market power. Highly concentrated markets may be contestable if there are no barriers to entry or exit, limiting the incumbent firm's ability to raise its price above competitive levels.
Marginal revenue For a firm facing perfect competition, price does not change with quantity sold (   P ′ ( q ) = 0  {\displaystyle P'(q)=0} ), so marginal revenue is equal to price. For a monopoly, the price decreases with quantity sold (   P ′ ( q ) < 0  {\displaystyle P'(q)<0} ), so marginal revenue is less than price (for positive   q  {\displaystyle q} ).