Check box based on value of another field

Your code is nearly correct, assuming it's a validation script on the text field where the number value is entered. 

the problem is that you are using comparison operators, instead of assignment operators. 

Here's the fix.

if (event.value >= 2500)
{
   this.getField("PriceBox").value = "Yes"
} else {
   this.getField("PriceBox").value = "Off"
}

Try:

// this is a field level calculation script that
// goes in the AMOUNT FIELD
// Assumes PriceBox is a yes or no checkbox field

var amount = Number(this.getField("AMOUNT FIELD").value);
var checkbox = this.getField("PriceBox");

if (amount >= 2500) {
checkbox.checkThisBox(0, true); // Check the box
} else if (amount < 2500) {
checkbox.checkThisBox(0, false); // Uncheck the box
}

event.value = amount; // Optional, if the field should still show the value