I've got 5k right here right now for it
2004 STI PPP with only 31k on the clock - if its genuine mileage with no faults and in half decent condition, i'd look to advertise at probably £8500, then taking an offer just over £8k. They've held their value well in the last 2-3 years and in some cases gone up.
Not sure where you got £5k from, thats WRX money for that age.