Often it encompasses how busy the service is, what the current collection percentage is, etc, then if you use their equipment etc. They then establish a quote usually a certain percentage of what they collect. Expect to pay about 8-12% of collections but keep in mind most of the times the amount collected will go up. For example one service I am aware of used to collect about $10000 per month doing billing collecting on their own. They started using a third party billing service that provided laptops with run report software and within a couple of months the service was getting about $25000 a month after the billing company took their cut.
Now there are also billing only services that charge a smaller percentage but they basically do not do anything but mail a bill or file for insurance. They usually take a 3-6% of what comes in but you will not see much more coming in than if you were doing the same in house.