Hi there and welcome!
Buying all of the tools, materials, proper switches and components that WON'T melt down (cheap China crap melts down on you), etc. Will cost more than an Auber PID.
Also fabricating on the stupid enclosures is not easy and finding good sized enclosures is not a simple thing unless you have the tooling to do it more easily.
If you have all that then you save about $20-40 vs buying an Auber and spend 1-3 days building one.
An SMF member
JC in GB
makes some for sell so you can speak to him about them. Hit him up and see what he can do for you :)
Lessons Learned building PIDs:
If you want to put in the extra money and like to do the project then feel free to give it a shot but you won't save money on the initial 2 builds. I've built a couple before and learned a few things like don't use project box outlets, instead canabilize 12-16AWG extension cord male and female ends for your plugs.
Buy a super heavy duty on/off switch like a 25-40A marine breaker box switch because all Chinese rocker switches I bought melted down. Buy a humana bussman fuse holder because other chinese ones will melt down
Drill as many holes as possible and use relief glands to fasten cables to the project box/enclosure.
Buy a large enough project box and enclosure and make sure you can ground in it.
You have to cut rectangles for the PID itself so have tooling to easily do it. I use a dremel roating cutting tool and its not great for cutting squares but gets the job done.