There were 3.0Si's going for £16,000 3yrs ago just before I bought my 3.0i... and they had average miles for age on them.
'OK' spec Z4 MC's were going for £22,000 around then too.
So all considered, they have actually done pretty well in the longer run I think.
Short term you may have been caught out by the whirly economy.
Iirc, an 'ok' 3.0Si was about £20,000 30 months ago, so assuming 50% depreciation every 3yrs, £15,000 sounds very good!
Dave