Nuts and bolts are far more expensive
But it depend how many nuts and bolts are needed. He cant be needing 1000s nuts and bolts for a bracket that the overal solution cost far more if rivet was to be used.
A few cent more for a simple nuts and bolts job wont hurt a one off project.
Of course a cheaper solution can be acheived if the bracket was bended/fabricated correctly at the first place