Don't have Javascript experience and after looking through numerous online articles I need help on the proper syntax for the following

"undefined" is probably not a good field name - I would use something that is more descriptive. For for now, let's go with "undefined". You will need to create a calculation script for the "undefined" field. This script needs to get the value from your "purchase" field and compare it, and then set the field value accordingly. This should work:

var f = this.getField("purchase");
if (f != null) {
  var v = f.value;
  if (v > 1000) {
      event.value = v/2 * 1000;
  }
  else {
      event.value = 2;
  }
}