My standard is to go to
www.cars.com and do an advanced search for the entire US of the model year I'm looking at. For '05 S models I see two clusters. Some around $36k and some around $28k. Next I consider that those are asking prices so I discount them 5-10%.
All this tells me I would pay no more than $29k for a very good condition car and the CPO would add no more than $2k to that (dealer cost IIRC is $1900 plus inspection cost plus fixes costs). I bought both my Boxsters for $4k each under what the online estimators said should be the price...in each case looking for a motivated seller.
Maybe your area of the country is higher. Of course since the car may be under water by now.....