$this->mongo_db->where('stock_type', 'used');
$this->mongo_db->where_between('internet_price', $amounts['bottom_amt_used'], $amounts['top_amt_used']);

db.vehicle_stock.find({ $or: [ { stock_type: 'used', internet_price: { $gte: 123, $lte: 145 }}, { stock_type: 'new', internet_price: { $gte: 161, $lte: 184 }} ]});

$this.mongo_db->where(array(
  '$or' => array(
    array( 'stock_type' => 'used', 'internet_price' => array( '$gte' => 123, '$lte' => 145 )),
    array( 'stock_type' => 'new', 'internet_price' => array( '$gte' => 161, '$lte' => 184 ))
  )
));