I can't render default values on form element in theme_table()
In FAPI (and renderable arrays in general), keys starting with a # are considered properties - so not renderable elements. This means their children aren't processed for content properly.

There are several methods to do what you want, but the easiest is probably references. In your specific example, it would look like this (I've trimmed out some stuff to make my changes more clear):

function _ss_menu_zone_test($form, &amp;$form_state) {

  $tablesort_headers =array(
    array('data' =&gt; t('Price'), 'field' =&gt; 'price'),
  );

  // render the tablesort of menu items.
  $form['category_' . $tid]['items'] = array(
    '#theme' =&gt; 'table',
    '#header' =&gt; $tablesort_headers,
    '#rows' =&gt; array(),
    '#sticky' =&gt; TRUE,
    '#attributes' =&gt; array('id' =&gt; 'zone-items-' . $tid)
  );

  $formfield = array(
    '#type' =&gt; 'textfield',
    '#default_value' =&gt; '10' // default value set
  );

  //We need to include the field in the form so it's rendered. This wont display because we're using #theme =&gt; table.
  $form['category_' . $tid]['items'][] = array('formfield' =&gt; &amp;$formfield);

  //These are your rows, note the byreference on the formfield.
  $tablesort_rows = array(
    array('price' =&gt; 'test row format 1'), // example of basic string row
    array('price' =&gt; array('data' =&gt; 'test row format 2')), // example of simple string but using data
    array('price' =&gt; array('data' =&gt; &amp;$formfield)),
  );

  //Add your rows to the table.
  $form['category_' . $tid]['items']['#rows'] = $tablesort_rows;

  //Since we used references we want to unset the variables we created.
  unset($formfield);

  return $form;
}