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I paid £11,950 for an April 04 with 15,000 miles 5 months ago. That was from a dealer (non-subaru). Auto trader has an 05 with 12,000 miles on now for the same price, again trade. I'd guess similar money on a forecort for yours but private sale about £11k.
When you can buy an unused WRX06 for less than 16K it's bound to destroy the values of these cars.
Add to that image problems, general undesireability and the fact the cars aren't all that quick compared to the competition you have a heap of reasons why an Impreza isn't a very good buy at this end of the scale.