Magento 2 – Get Custom Value from Custom Table in Product Grid

I have create custom module and render product grid with custom column "Store Price" and that value is saved in custom table with product id. Now I want to render store price in that coulmn.
Note: that custom column is rendered from "render" option with Block path in addcolumns.

Prepare columns:

    protected function _prepareColumns()
    {
        $this->addColumn(
            'entity_id',
            [
                'header' => __('Product Id'),
                'sortable' => true,
                'index' => 'entity_id',
                'header_css_class' => 'col-id',
                'column_css_class' => 'col-id'
            ]
        );
        $this->addColumn(
            'name',
            [
                'header' => __('Product Name'),
                'index' => 'name'
            ]
        );
        $this->addColumn(
            'sku',
            [
                'header' => __('Sku'),
                'index' => 'sku'
            ]
        );

        $this->addColumn(
            'price',
            [
                'header' => __('Price'),
                'index' => 'price'
            ]
        );

        $this->addColumn(
            'store_price',
            [
                'header' => __('Store Price'),
                'index' => 'entity_id',
                'renderer'  => '\Vendor\Modulename\Block\Adminhtml\Template\Renderer\StorePrice',
            ]
        );

        $this->addColumn(
            'product_id',
            [
                'header' => __('Select Product'),
                'sortable' => false,
                'type' => 'checkbox',
                'field_name' => 'product_id[]',
                'index' => 'entity_id',
                'header_css_class' => 'col-id',
                'column_css_class' => 'col-id'
            ]
        );



        return parent::_prepareColumns();

Render File:

namespace Vendor\Modulename\Block\Adminhtml\Template\Renderer;
use Magento\Backend\Block\Widget\Grid\Column\Renderer\AbstractRenderer;
use Magento\Framework\DataObject;
use Magento\Store\Model\StoreManagerInterface;

class StorePrice extends AbstractRenderer
{
 private $_storeManager;
 /**
  * @param \Magento\Backend\Block\Context $context
  * @param array $data
  */
 public function __construct(\Magento\Backend\Block\Context $context,   StoreManagerInterface $storemanager, array $data = [])
{
    $this->_storeManager = $storemanager;
    parent::__construct($context, $data);
    $this->_authorization = $context->getAuthorization();
}
/**
 * Renders grid column
 *
 * @param Object $row
 * @return  string
 */
public function render(DataObject $row)
{

    return '<input type="text" maxlength="10" name="product_price[]">';
}
}

Thanks in advanced