You have to take a deep breath to get to the point of this sentence! The core idea is that all the factors listed in the long dependent clause pale by comparison to the real problem, which is the huge US market. I realize that my suggetions don't all mean the same thing, but they are all intended to subordinate the laundry list of other factors that are but peanuts compared with the main problem.

"Leaving aside the burgeoning growth of synthetic drugs and the ups and downs [volatility of the market] and passing fads, such as Colombian heroin in the 90s, it is the stability of the mammoth consumer market to the north that is the most ...."

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Note added at 2 hrs (2008-04-13 19:40:58 GMT)
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This is a variation of the old "sin perjuicio a" - I used to have a card in my desk listing about 25 ways to handle this structure in English