There are plenty of threads on this forum on this subject, not sure which search terms would work to pull them up.
Nutshell version of forum wisdom is the best clay lube is a quick detailer product because it lubricates the surface while you're claying and after you're done claying and wipe the residue off the paint looks good because quick detailers are made to mist on paint, then be wiped off and leave the paint looking good.
This is something that is important to most people but they don't know it, the reason we know this is because in all our Saturday classes, when we work on a demo car, if while claying we leave any residue or a clay smudge people panic and get all worried about the paint. Then we wipe the paint again and the smear or smudge is gone and they calm down.
So we know that after using a product, (any product really), to work on paint, especially a product from Meguiar's, it's important that the paint looks good after its use.
Now let me tie this in to your option, Meguiar's doesn't recommend using car wash soap mixed with water as a spray lubricant for claying, the clay can be affected in a negative way by the soap and soapy water is not designed to be sprayed onto paint, and then wiped off to leave behind a a nice looking shiny paint job. This circles back around to what we typed above. That is, soapy water used as a clay lubricant isn't going to wipe off as easy nor leave your paint looking good after you clay because it's soapy water, not a quick detailer.
Besides all that trivial stuff, we understand that in some countries a bottle of Quik Detailer can be quite expensive and our customer's treat it like gold and would prefer to save it for wiping the car down to remove light dust and restore the 'just detailed look' and thus like you, search for a lest costly substitutes for use as a clay lube.
So just to be clear, Meguiar's does NOT recommend using water with a little car wash soap mixed into it as a clay lube, but that's what some people will use when looking for a less costly substitute.