I recall reading that somewhere (maybe Pottermore?), Rowling said that the Malfoys gained their great family wealth by trade with muggles, until the establishment of the statute of secrecy, at which point they were quick to join the "Yep, we knew those dirty muggles just wanted to exploit and burn us all along!" crowd. I don't remember if there is any similar detail about other wealthy families in canon or MoR; there's Flamel with the Philosopher's Stone, and the Potters with a combination family inheritance/bounty on Voldemort, but the Malfoys appear to be decidedly the richest family in Magical Britain, and I'd imagine that even with a 400 year gap, being those to most thoroughly exploit trade with muggles would be more than sufficient to explain their success.