I think you should look at what the parts cost you new, then look at what equivalent parts cost these days new, then keeping those figures in mind, deduct an amount based on age. The price when new, however, shouldn't carry as much weight (the P3 might have cost a few grand, but even a brand new pc these days is only a couple grand). I'd say a fair estimate would be to half the price every 2 or 3 years, depending on the condition of the item.
Those Creative speakers: Are they the Cambridge Soundworks? I have a set of those and would LOVE another set for my other box!