This really depends on a lot of variables. How you charge is a preference, some do %, flat fee, per claim, etc. If your looking for averages that varies depending on your area, I couldn't even begin to give you an average. i bill a flat fee based on my time. A % really only benefits the provider and not you. A per service fee is hard to sell as well as track because there are so many things a biller does to build a claim and you can outprice yourself with an 'ala carte' model.
You also want to research your state if you plan on a % as some states prohibit providers from entering into contractual relationships where there is fee-splitting. In addition to your state you would also need to research the state a provider is in if you plan to bill for out of state clients. That's why it's good idea to have a few pricing structures laid out. In most cases they will be different depending on the clients specialty, location, volume, type of carriers, and of course the work involved.