Easy: create a formula called Billed and a formula called Free. In the Billed formula do the following:
If Incident.SLA = "Billed" then 1 else 0
For the Free formula:
If Incident.SLA = "Free" then 1 else 0
Put these fields into your detail section. Then insert a summary on each of the formula fields. Then you will have the number of instances where the field said "Billed" and the number of instances where the field said "Free:. There are probably a dozen ways to do this, but this is the most straightforward one I could think of.