A. For a UK citizen, on tourist and business visas, normally they grant a two year visa (subject to Consulate discretion). With a two year visa, they can stay up to 90 days at a time. Chinese embassy issues visa for single entry and multiple entries and once it has been issued you have to apply for a new one next time. The validity or duration of time limit cannot be extended regardless of the number of entries. The visa is valid from the day stamped and number of days a visa holder is permitted to stay is counted from the day visitor enter in China.
The cost of China visa is entirely depends of number of entries and type of visa you want to acquire. For more information regarding China visa cost, feel free to browse the website.