When Sideshow became the exclusive distributor for Hot Toys in the US, it created a monopoly, which very quickly lead to a significant rise in prices for Hot Toys figures in the US - far outpacing inflation, rise in supply and material costs, and the pricing of the same figures abroad. Great for Sideshow, bad for third party retailers, horrible for consumers.
This is of course almost always the end result of monopolies, due to the reduction of competition and consumer choice. When a single distributor inserts itself as a middle man, every retailer has to pay that extra "middleman tax" to get the product in stock, which limits their ability to provide discounts to consumers.
Of course, there have been many factors that have lead to the rise in Hot Toys prices and 1/6 figures in general, over the years. I'm not saying that they'd still be at 2009 prices if Sideshow hadn't become Hot Toys' exclusive distributor. But they would almost certainly be lower than they are now, had that never occurred. (If you are a newer collector, I would advise against looking at the prices were were paying 10-14 years ago - it will certainly dishearten you!)