I have built post and beam and stick built structures. Pole barns are not the same as post and beam. Pole barns are inadequate for a shop, machine, wood etc. Ceilings are too high, insulating is a problem and siding needs a lot of preparation to make it look straight. I have sided pole barns but not installed the poles and timbers. Because the poles are not in a line due to the difficulty of setting poles in a hole and in a line.
If the poles are set fairly close together then the horizonal 2x4's may need minimal packing to get it straight.
Often the poles are 10'-0" or more apart. This presents a problem for the siding crew. I installed vertical 4x6's in between the poles and horizontal members nailed to it. Bottom of 4x6 bolted to concrete floor with an angle iron. Top of the 4x6 done similarly.
I do not know what the cost would be as I did the siding and framed and sheathed the roof only.
My own shop is stick built in 1997. It is 20'x30' and 8-0 ceilings. I sided the studs with 7/16" OSB inside and out. Inside I screwed the smooth side to the studs. I never painted the walls as the rustic look appealed to me. I have 9 windows on the first floor so I get plenty of light.
I have 120 volt and 220 volt receptacles on the wall and two on the floor. Cost was $9000.00 at that time, I recall 7/16" OSB around $5 /$6 then.
Naturally you can't compare costs from 1997 til today.
Post and beam construction is fine too. I have worked on historical homes that were framed this way. Studs were odd sizes ,about 2/3" x 4/5" .
Studs had a tenon that went into mortise and held tight with a draw peg.