Here are some examples. They would go in the calculate event of your amount field
Say there's a $5,000 threshold for the field to be visible.
if (this.rawValue > 5000) field1.presence = "visible";
else field1.presence = "hidden"; //to rehide if the amount drops below your threshold.
If there's a second threshold for the other hidden field like $2,000 for field1 and 5,000 for field2.
if (this.rawValue > 5000) field1.presence = field2.presence = "visible";
else if (this.rawValue > 2000) {
field1.presence = "visible";
field2.presence = "hidden";
}
else field1.presence = field2.presence = "hidden";