I'll guess no more than low $40s.. one challenge it seems is that everyone recommends buying either CPO (and at 2005, its at the end of being CPO able I would think) or an aftermarket warranty (which likely costs $3-4K).
The cheapest Porsche to own is not necessarily the cheapest Porsche to buy.
Take you time here, or rennlist or even the 6speedonline forums to learn about whats good and bad about the 997 in general and the early cars (2005) in particular.