how much does Hong Kong visa cost for Afghanistan passport and what is the requirements?

Which type are you going to apply? The cost might be different according to the type you apply for.

I am going to apply for L type.

Shahab, I think you have to apply from the HK Immigration Department directly. A transit type costs HKD 84. An ordinary type costs HKD 160. Different types have different requirements. Which type are you going to apply?

i need a ordinary type visa for HK what is the requirements

In order to apply for the tourist one, you need to prepare the application form with recent photo, your bank statement, photocopy of the applicant's travel document containing personal particulars, date of issue, date of expiry, round trip ticket to HK and hotel booking confirmation letter.

The fare is around 160HKD for applying for tourist one.

If i book tickets from other country will there be any problem

It is ok.