subtraction

I've been trying a JavaScript that computes the remaining balance. 

The customer picks a package which assigns a price to the subtotal and when the customers enter a deposit it should generate the remaining balance.  I've tried this javascript

var t = getField(“Price”).value;

var d = getField(“Deposit”).value;

event.value = t - d;

but it is not working.

Is there anything wrong with the JavaScript?  Please help!!