You can use the "seed" functionality to do this pretty easily. When you set a field as the "seed" for a function then that field becomes the oField in your custom function. If you do not set a seed field then it is assumed that you want the current field to be the oField value.
So you would need to create a function similar to the last one here and assume that oField is your amount field (because you will set that field as the seed column). Then the logic is pretty straight-forward, just replace = "" with >= 1000 and you should be most of the way there!