Calculate cost based on chars in text area

I think the following code snippet will point you in the right direction. My form had a textarea, a hidden product field and a total field.

<script type="text/javascript">
jQuery(document).ready(function(){
	//NOTE: replace input_91_3 with the ID of your textarea
	jQuery("#input_91_3").change(function(){
		gformCalculateTotalPrice(91); //NOTE: replace 91 with your form ID
	});
});
function gform_product_total(formId, total){

  //only apply logic to form ID 91
  if(formId != 91)
	return total;

  var price = 0;
  //NOTE: replace input_91_3 with the ID of your textarea
  var character_count = jQuery("#input_91_3").val().length;

  if(character_count < 60){
	price = 7;
  }
  else if(character_count < 80){
	price = 8.5;
  }
  else if(character_count < 100){
	price = 10;
  }
  else if(character_count < 120){
	price = 11.5;
  }
  else if(character_count < 140){
	price = 13;
  }
  else if(character_count < 160){
	price = 14.5;
  }

  //NOTE: replace ginput_base_price_91_2 with the ID of your hidden product price field (you can see it by inspecting the markup of the form)
  jQuery("#ginput_base_price_91_2").val(price);
  return price;
}
</script>