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Hello! I am wondering why is the cost of pin joint more expensive than the cost of welded joint? I learnt from my lectures that fix joint are usually more expensive to construct right?
Also, what do you meant by truss being more efficient than beam in carry large distance?
Generally speaking, welded joints on trusses are more straightforward to fabricate (there is just less fabrication involved) and as a results they are typically cheaper - again this refers specifically to trusses, which often have elements framing together at a range of different acute angles.
However, if you are fabricating a beam to column connection in a steel frame, for example (which is what I suspect your lecturer may have been referring to), then a pin connection will be cheaper than a fixed connection (which may be achieved by welding, although not always).
In terms of structural efficiency, a truss will have a larger strength to weight ratio than a beam that spans the same distance - as such it uses material more efficiently.
Hope that helps.
Seán