there are a number of plans out there for much nicer jigs that probably don't cost that much. You can get aluminum extrusions from Misumi for a very reasonable price. Apparently nobody has ever felt like offering a kit though, that is one disadvantage. I'm guessing a lot of these jig kits don't get built.
Before buying anything, I would create a list of things a jig might do for you, such as:
Be easily adjusted
Be easily positioned for hot work, so you can have parts at any angle you want so gravity is your friend
Have sufficient offset, and yet be sturdy enough that offset does not compromise function.
Be reasonably able to handle the range of designs one want to create.
Easily related design parameters to jig position. This can be a big time waster. The Paterek method works, but is quite time consuming to relate.
Define dimensions and fits
Be adapted to your form of building
Fix parts firmly enough that they do not shift during reasonable hot work
Contribute to the professional level of your shop where relevant
Aid in designing
Support purging, heat sinking, grounding, whatever...
Then ask yourself whether a particular design will do whichever of these or others, you want. I have 5 basic requirements, and most of the jigs I looked at were pretty close to zero on them. The jig I eventually bought thinking it would handle my 5, ended up not doing it, it only hit 3. For instance it was too short for some of the frames I wanted to build, it had never occured to me to ask, since I assumed it would build something other than track bikes...
A lot of jigs are just jigs, the don't actually do anything.