You can use below calculated field with below definition to get what required. Although I would suggest to do this in sql with case statement. Which give better flexibility.
ifelse(toString(extract(‘WD’,{you_date_column}))=‘1’,0.00,toString(extract(‘WD’,{date_val}))=‘7’,0.50,1.00)
here quicksight resolves Sunday to 1 and 7 for Saturday