Keep a running total sheet.
Include everything to make the manifold.
Time
Welding material
Flanges
Metal piping
Freight(s)
Electricity
Hole saws
Grinding disc
Fuel
Track it all you'll be surprised what it will cost in the end. No doubt if you have the fab skill you can build a setup.

If done right it will looks simple.
Simple looking takes time to lay out.
Where to start where to end the part. It take time trial and error.