Thanks for the kudos, chillinn.
I know you are just having some fun, but undoubtedly the OP just wants a battery tab welder, as anyone who makes battery packs should have.
In the thread you linked, the welding of the batteries was a lesser part of the project. The biggest part of it, at least for me, would be the construction of the physical structure of the pack, which has some pretty unusual requirements. Here it sounds like the tab welding is pretty much the entire project.
The cheapest complete tab welding system I have seen is around 140 USD, and it is REALLY cheap. It's available on USAbanggood, and probably under other names at other vendors. Proper professional ones start at ten times that and go up from there, and that's just the basic unit, without leads, hardware, etc. I expect the chinesium ones would work reasonably well until they don't work at all. I don't know that I'd want to invest that much money in something so questionable. But I'm a cheap ba****d.
I've seen a number of DIY jobs that range from far more cringe-worthy than the $140 chinesium ones to reasonably respectable performance. Some of the reasonable ones look like they cost less than $30-40, but they take a little bit of skill both to build and to operate, and more than a little time to build.
Recently I realized there are a bunch of units on flea bay that are semi-DIY. These are intended to attach to a (VERY) high current (but remarkably small) LiPo cell, or a large and somewhat expensive capacitor bank, or even a good sized lead-acid battery, and otherwise are pretty much complete. They are FET based controllers and are remarkably inexpensive, ranging from $25-40. Theoretically, they should give pretty good results with a little practice. Clearly they are not made for volume production, but for a few packs here and there, they might be perfectly adequate.
Unfortunately, the bulk of the cost is in the power reservoir. The two 3000F supercapacitors recommended by one unit range from $33-95 each in my quick search, depending on what you get and where you get it. You'd need to add a 5V power supply on top of that, but that should be no more than a few bucks. I didn't explore what a 300-500A rated LiPo would cost, or a charger. A lead-acid battery suitable for some can be had for $45, possibly less. Think tractor or motorcycle battery.
A good way to learn about these things is to google 'battery tab welder', go to 'images' and start clicking and reading (or watching).