Connecting a FireWire 800 external hard drive to MacBook Pro early 2013 through its USB 2 port
The Apple Thunderbolt to Firewire adapter is $29, and I don’t think you’ll find much cheaper that that for adapter cables. However if the external Firewire drive is just a normal drive in a Firewire enclosure then you could pull the drive from it and use either a USB3 hard drive case or even a laptop hard drive SATA to USB3 cable. Those cables and enclosures start at around the $10 mark on big ecommerce sites.