How to auto fill a text field?

[eluser]glenconde[/eluser]
Hi guys, please help me.. i wanted to "Auto Fill my textbox" from a computation of another textbox.. but i can't get it.
anybody help please.

thank's!

this is my code in controllers

Code:

function save($item_id=-1)
{
  $item_data = array(
  'name'=>$this->input->post('name'),
  'description'=>$this->input->post('description'),
  'category'=>$this->input->post('category'),
//  'supplier_id'=>$this->input->post('supplier_id')=='' ? null:$this->input->post('supplier_id'),
  'item_number'=>$this->input->post('item_number')=='' ? null:$this->input->post('item_number'),
  'cost_price'=>$this->input->post('cost_price')*(.30)+('cost_price')=='' ?null:$this->input->post('unit_price'), [i]<---- [b]the Result of this here i want to post to unit_price textfield[/b][/i]
  'unit_price'=>$this->input->post('unit_price'),
  'quantity'=>$this->input->post('quantity'),
  'reorder_level'=>$this->input->post('reorder_level'),
  'allow_alt_description'=>$this->input->post('allow_alt_description'),
  'is_serialized'=>$this->input->post('is_serialized')
  );