Express Z/work visa in China

Hi there,
I must start work for an American university in China on September 1, 2014. I'm waiting for my documents from the US/China for a work Z. On your site you state that it is issued for 7-10 work days. In the application you give an option for obtaining it by express service. My question is what is the time and the fee for an express service (I'm a Bulgarian citizen). Thanks.