You can never tell with these things.
If I were to GUESS, I would say they they start out high, level off a bit, maybe go down in price for awhile, and then start appreciating again in the future do to "Rarity".
The bottom line is what is the set worth to you?
That's true that the price will go up in the future. I took hard lesson with Guardian. It's my first predator and I had no idea about price was too high and I ought it coz I though it would only go up. Look at the price now. But, I know it will be more expensive in the future.
As for scarecrow set, there was only TOnyCheung guy sold it for $999. No one bought it now he is selling the set at $600 and I don't see any buyer as well. Now look at ebay, more sellers are selling it. Well, I say 4000 pieces not really that rare. There are only couple hundreds freaks here so once all scalpers start selling this set the price will be much lower.
When all sold out then when Scar, Celtic, Chopper scenarios kicks in
If you have the money and the figure worth more to u then go ahead and buy the set.