What is the average billing rate for billers? I am charging my clients 7% of what gets paid. One of them is asking for a decrease stating my rates are too high. What do you all charge?

Without going into exact figures or percentages, here is my stance... the other option for them is to go elsewhere and get cheaper! Next time this client tells you your rate is too high... have your blurb, and horror stories ready, and scare the willies out of them about "cheap services".
Don't be smart... just have a real conversation that serves as an eye-opener... Tell them stories of business failure, and the cost to fix things, and catch up. Somewhere in there, throw in the phrase "Of course, if you want cheap, you can get cheap; and there is always India!". Also, mention that it they should decide to come back later to fix the mess, your rates will be likely higher, especially since the 7 per cent they are enjoying now is a special rate that was established especially for them as an appreciated customer. Once you painted the picture, I am sure they will be happily staying.
I've used this technique myself in web design. It works. Never ever heard as much as a peep of a complaint ever again, even though over the years my rates still increased a little. My biggest advantage and selling point (hook) is my reliability, accuracy, credibility, productivity, and availability within 1-2 hours, and on weekends, if needed, to help or answer questions. Sadly, I too had to literally convince a couple clients, that my rates are a BARGAIN... and right on target.
I NEVER go back on my rates! They were carefully determined for each individual client before a proposal was forwarded and discussed again before the contract was signed. Agreed, is agreed... Sometimes, however, if I feel the customer is sincere, and valuable to me, I am willing to sit down and examine the breadth of the current services to determine if we can pin point certain parts of the service that they no longer need; or can take over themselves, and thus, rather than reducing my rates, make every effort to meet their new budget.
However, if I sense it's simply a case of "buyer's remorse" ... a rate reduction for the services agreed to is not up for discussion. Once agreed, I deliver, and expect to get paid. However, I am speaking of web design rates and contracts, rather than billing services. Nevertheless, I am fairly sure, the situation is similar.