function validateLine(context){
    if(context.sublistId == "custpage_fee_lines"){            
            let form = context.currentRecord;            
            let Price = form.getValue('custpage_price');
            let Amount = form.getValue('custpage_amount');
            if(!isNull(Amount) && !isNull(Price)){    
                let cnt = form.getLineCount('custpage_fee_lines');
                let total = 0;

                let rate = form.getCurrentSublistValue({
                    sublistId: 'custpage_fee_lines',
                    fieldId: 'custcol_fee_rate',
                })
                
                console.log(Amount, Price, rate);

                form.setCurrentSublistValue({
                    sublistId: 'custpage_fee_lines',
                    fieldId: 'custpage_fee_calced',
                    value: rate * Amount * Price,
                    ignoreFieldChange: true
                });

                for(let i=0;i<cnt;i++){
                    form.selectLine({
                        sublistId: 'custpage_fee_lines',
                        line: i
                    });
                                                                                        
                    total += form.getCurrentSublistValue({
                        sublistId: 'custpage_fee_lines',
                        fieldId: 'custpage_fee_calced',                                                        
                    })
                }
            }
        }                 
    return true;
}

validateLine here

last I recall, SuiteScript 2.1 still had serious issues with client scripts. i would use 2.0.