The raw material cost for steel is by the ton. Barring unusual circumstances. And metal bents are very very structurally efficient.
Incidentally, they are not “I beams” and technically I-Beams are Wide Flange Shapes and used vertically as well as horizontally, and designated with as W(n)x(m) where W means wide flange, n is the depth in inches, and m is the weight in pounds per lineal foot. W shapes are rolled in rolling mills.
Bents are fabricated from steel plates to a design specific to the structure.
No commercial solar farms use large members to save on labor costs.
Standard ground mount equipment also uses pre-engineered metal, just of much lighter weight due to supporting a small fraction of the load.