Multiply of two columns in yii2 - yii2

Question

I have two tables -
1. rmtemplate
id, productname, rmname, qty
2. rmname
rmid,rmname,min,unitcost
The query I want to run is -
SELECT id,productname,rmtemplate.rmname,qty,rmtemplate.qty * rmname.unitcost as cost
FROM `rmtemplate`
left join rmname on rmtemplate.rmname = rmname.rmname
In Yii2 I've joined two tables with no problem. The unitcost is showing without any trouble. But when I try to multiply rmtemplate.rmname to rmname.unitcost it returns error - 'Getting unknown property: frontend\models\Rmtemplate::cost'
RmtemplateSearch.php -
public $cost;
public function rules()
{
return [
[['id'], 'integer'],
[['productname', 'rmname', 'qty', 'unitcost','cost'], 'safe'],
];
}
$query = Rmtemplate::find()
->joinWith(['unitcost'])
->select(['id','productname','rmtemplate.rmname','qty', '(rmtemplate.qty * rmname.unitcost) as cost']);
Rmtemplate Model
public function getUnitcost()
{
return $this->hasOne(Rmname::className(), ['rmname' => 'rmname']);
}
Rmtemplate index.php
<?php
use yii\helpers\Html;
use kartik\grid\GridView;
use kartik\select2\Select2;
use yii\helpers\ArrayHelper;
use frontend\models\Rmtemplate;
/* #var $this yii\web\View */
/* #var $searchModel frontend\modules\rmprod\models\RmtemplateSearch */
/* #var $dataProvider yii\data\ActiveDataProvider */
$this->title = 'Rmtemplates';
$this->params['breadcrumbs'][] = $this->title;
?>
<div class="rmtemplate-index">
<h1><?= Html::encode($this->title) ?></h1>
<?php // echo $this->render('_search', ['model' => $searchModel]); ?>
<p>
<?= Html::a('Create Rmtemplate', ['create'], ['class' => 'btn btn-success']) ?>
</p>
<?= GridView::widget([
'dataProvider' => $dataProvider,
'filterModel' => $searchModel,
'columns' => [
['class' => 'kartik\grid\CheckboxColumn'],
'id',
//'productname',
[
'attribute'=>'productname',
'filterType'=>GridView::FILTER_SELECT2,
'filter'=>ArrayHelper::map(Rmtemplate::find()->orderBy(['productname' => SORT_ASC])->asArray()->all(), 'productname', 'productname'),
'filterWidgetOptions'=>[
'pluginOptions'=>['allowClear'=>true],
],
'filterInputOptions'=>['placeholder'=>'Charge Name'],
],
'rmname',
'qty',
[
'attribute' => 'unitcost',
'value' => 'unitcost.unitcost',
],
'cost',
['class' => 'kartik\grid\ActionColumn'],
],
]); ?>
</div>
Please let me know if any further input required.