Yes, the al mounting rails are so expensive,guess it,s a special extrusion ,used by only solar installers.
I looked at the purlin type "tophat" extrusion. I think it,s very strong ,you can order it in different sizes and thickness. Mine is 7.2 mtrs long in one length-thats another cost, getting it delivered.too long for your car/trailer.
Then you have to be fairly accurate in your location of fixing points -you don,t want holes drilled all over the place. thats where the real stuff is best,I guess.
when you have tradies wanting $400-500 per day wages ,thats why they use the quick fixing stuff.
Bruce