I wish to visit Shenzhen and Guangzhou on a tourist visa, What requirements and what's the cost?

I am a British citizen with residence permit in Qatar valid until February 2012. I have a flight to Hong Kong on the 25th August and I wish to visit Shenzen and Guangzhou on a tourist visa. Can I apply through you? What is the cost of a single entry L visa? Do I need any documents other than my passport and Qatar ID?