The only barometer I have ever really known is what the average selling price around the net (ie. here and Ebay) is.

The biggest problem I've seen is people will sell, say, a 20GB for $500, but want $750 for a 30GB. Now I'm no rocket scientist, but I think I can get 10GB of laptop drive for less tha $250.

If I were to sell mine (let's say if I needed to pay for a heart transplant or something), I would probably ask for:

$350 base, just for the unit itself
+$380 since that's todays low on pricewatch.com for 2 x 80GB laptop drives. (My MkIIa is a 160GB)

for a grand total of $730. Maybe that's low, who knows. But that's what I'd charge. Or they could have it for $400 with a 10 or 20GB in it. Whatever I grabbed first.

But, like I said, it would require VERY desperate times for me to sell.