[Tom Goldman]

I usually use the 33% figure as a rule of thumb, since most cars done in the last 20 years have .134"wall ERW tube instead of .120 DOM
Average chrome moly roll cage weighs around the same as a .134" wall mild steel roll bar.
The added safety from a cage in a car like a shoebox Nova is worth the price and even a few pounds.
It's not just how fast you are going any more, you have to consider the 150+mph bullet on 9" tires in the other lane .