If the site has been configured to use an enrolment method that requires payment (such as the Paypal module), then you can enter a course cost here with no symbol (the currency is set by the enrolment plugin). For example, 19.95.
If the cost field is empty then the payment option is not presented and the interface will fall back to other methods of enrolment (such as keys, or manual enrolments).
If the cost field is NOT empty, then students trying to enrol will be presented with the option of making a payment to enter.
If you ALSO enter an enrolment key in the course settings, then students will also have the option to enrol using a key. This is useful if you have a mixture of paying and non-paying students.