as i understand:
the setting "enrol_cost" in paypal-plugin should be used, if a course has no cost set.
but it doesn't.
the error is in enrol/paypal/enrol.php
where this is checked on line 25 and 95:
if ( (float) $course->cost < 0)
// use default cost
// use course cost
but if i add a course it has the cost of 0.
so the solution is to change the if-statement to check for less or equal than 0:
if ( (float) $course->cost <= 0)