Pop-Up Window Based on Value of a Textfield

Hey Everyone,

I have a textfield entitled "GrandTotal" and when the value is $10,000 or more I want a pop-up window to appear notifying the user that they have to fill out an additional form.  Here is "GrandTotal" line of code right now:

<p>Grand Total*:
    <label>
      <input name="GrandTotal" type="text" id="GrandTotal" onchange="MM_openBrWindow('../PopUpSJF.asp','SJF','')" onblur="this.value=formatCurrency(this.value);" size="25" />
      </label>
  </p>

I created the onchange behavior; however it is set to create the pop-up for any change in the grand total field.  I also have the value to be put in currency format.

Does anybody have any suggestions?

Thanks,

Paddy