do i need to pay for visa?

You need to apply for a Visit/Transit type for joining the fair in HK and of course, you need to pay for them.

You can apply for it through your nearby Chinese embassy or apply directly at the HK Immigration by mailing your documents to them. The required documents are an application form with a recent taken photo, your valid passport, your bank statement, your hotel booking confirmations in HK, an invitation letter from your that fair and round trip tickets to HK.