OWC Mercury On-The-Go Portable 2.5" FW800/USB3.0 Enclosure Kit for Serial Sata
Still pricey, but… https://eshop.macsales.com/item/OWC/MSTG800U3K/? $58.00
OR: $47.25 (includes shipping) https://www.ebay.com/itm/125090840913?
OK, I hate to muddy the waters, but it usually pays to be goal oriented when making the decision of which FW external enclosure to get; in other words, what solution is the FW enclosure facilitating ? A quick data transfer ? A permanent Backup drive ? A system clone/setup mechanism, etc.
Well, as is true with many components and peripherals, it is extremely rare to hit the "cheap" category (as far as inexpensive) and the "works awesome" bullseye at the same time... more often, "cheap" means... inexpensive and built like crap. So, depending on the solution, it may not pay to go for quality, or you may shoot yourself in the foot if you go something that costs little, but is not reliable.
As far as the original question that created this topic, the OWC Mercury On-the-go falls way short in the inexpensive category. But, the OWC Mercury On-The-Go USB 3.0 & 2.0 / FireWire 800 (FireWire 400 Backwards Compatible) Enclosure Kit is by far the best in the single SATA HD/SSD category as far as quality. I have (4) of these that I use constantly, and the heat sink works well when transferring client's data to/from legacy macs that take many hours, it has the bonus feature of power switch, and that comes in handy without having to unplug it to turn it off during a job (I can't think of any other self-powered single FW 2.5" enclosure that has a switch). It is a work of art in quality workmanship and functionality...
Sleek, impact-resistant acrylic enclosure
Bus-powered, no AC adapter needed
Quiet, fanless operation
An aluminum heatsink and passthrough cooling vents keep your drive cool
This enclosure may be way overkill, if your solution is a one time data transfer or if it's not gonna get much use. For the "cheap" solution, buy a failed or ancient $10 VST or iOmega external FW drive (research if you want IDE or SATA as a drive interface); most will be a real pain in the ass taking apart; be ready to have some duct tape to re-assemble if that is even needed for your goal.
As many of you know, I have a lot of legacy FW stuff laying around, and it has always been trial and error with all these devices. For our site, all backups are done to (2) FW Mini Dual aluminum OWC setups; one at home, one in the office. For most jobs, I have switched to the "On the go drives." For really quick jobs, I use one of a box full on VST drives that permanently dis-assembled and used as drive trays.
For mission critical client backups from old G4s I use Newertech Gaurdian Maximus small RAID setups, that (2) out of (4) now have LCD issues, so I used to really think they were great quality, but now I realized why OWC sold them cheap. There is no reason the LCD readout should fail with such low use hours. All the external FW drives I mentioned here work well for legacy macs as far as FW chipsets, so just choose according to your needs.