MeAm Lo'ez

43:3  G-d further reminds them how He saved their firstborn when all the Egyptian firstborn were killed.  The Egyptian firstborn were indeed their "ransom," because Jewish behavior was no better than theirs.  He also reminds them how Egypt's treasures, their "ransom," together with those of Kush and Seva, fell before them after the plague upon Sancheriv's troops (37:36)