$total = 0;

        foreach($info->offer as $offer)
        {
            $total += $offer->{'exchange-rate'};

            echo $offer->{'exchange-rate'} . " " . $total . "<br/>";
        }

tulostaa:
0.016 0
0.016 0
0.016 0
0.016 0
0.016 0
0.016 0
0.016 0
0.016 0
0.016 0
0.016 0