Say you have an idea of what the demand curve looks like. For the sake of argument, let it be (100-p), so 100 people would want it for free, and none would want it at $100.

If your marginal cost is $50, to maximise profit you'd set the price equal to $75 and make $625 net.

If your marginal cost is $40, you'd set the price at $70 to maximise profit and make $900 net.