Javascript not calculating properly

I am new to javascript. I have a PDF form that adds all the total services [TOTAL SERVICES COSTS]. The next field is TOTAL SERVICES COST 2, where I have used a javascript to deduct 10% for pre pay. It is in the custom calculation field and reads [event.value = Number(this.getField("Total Services Costs").valueAsString) * 0.9]. It is not calculating correctly. Any help is appreciated.