Service tax on whole amount or the amount minus the expenses

Hi guys. 

                 i just watned to ask if the service tax will be calculated in the whole amount of payment to be recived or do we minus the expenses of a service provider. 

For example if the total payment coming in is £ 100.00

ANd the service provider has incurred £ 80.00 worth of expenses in providing this service. 

would the whole £ 100.00 be taken into considerstaion for service tax?

OR

Would the £ 20 would be considered for service tax?