Some datacenters in Hong Kong have a direct link with mainland China (ISPs) such as China Telecom (中国电信) and China Unicom (中国联通). It's important that your server (and in your case your "sub-account" on my server) be located in one of those datacenters if you anticipate most, or many, of your site visitors will be from mainland China. It is not more expensive per se, but you should be aware of it when you choose a hosting/server provider in Hong Kong. Many people choose to NOT get a Chinese internet content provider license, which also requires that you have both a business license in China and someone to make a personal appearance from your company at a government office during the ICP website license application process.
It is indeed faster to host within mainland China, but if you choose your server and data center location in Hong Kong correctly: it will provide most web apps with more than sufficient speed and responsiveness to service local, mainland-based Chinese customers/users/visitors to your website. Additionally, you can always turn things up a notch and go for your ICP license later when you've got more substantial loads/stress on your web app/site. By choosing right when you host in Hong Kong, you also have virtually no chance of getting your site or your server blocked from in China because you'll be on a "box" (this is geek-speak for your server/computer) that hosts Hong Kong businesses that can't afford to be blocked (which raises another point: please don't ask me to host (or post it yourself through your backend management console) any content that is critical of the local authorities).