Complex sentence: The "cast stone" was created onsite and patterned randomly to prevent repetition, deemed to be more economical than hauling quarried stone, but were just as difficult to install at 400 lbs each.
Split into simpler sentences: The "cast stone" was created onsite and patterned randomly to prevent repetition. This was deemed more economical than hauling quarried stone, though the blocks were just as difficult to install at 400 lbs each.