All of you if statements should have == instead of =. The 'i' in If needs to be lowercase. Try this:
var nTerm = this.getField("term").value;
if (nTerm ==24)
{event.value = 15.6}
else if(nTerm == 36) {event.value = 21.6}
else if(nTerm == 48) {event.value = 26.4}
else if(nTerm == 60) {event.value = 30.0}
else {event.value = 0}