I guess it's all supply and demand.
Here's a good example which I've been involved with...
My aunt has just had to sell her A4 2.0 Tdi after getting a job under a mile from home (to prevent any problems with DPF's and so on). And she has bought a 2.0 TFSI A4 (yes they actually sold a petrol one
) which is very similar and slightly better equipped with 30k less miles and in better condition.
She sold the higher mileage TDI for £6700
Bought the TFSI for £5700
The TDI sold in under 24 hours!
It's a pain to have to pay a premium, but if you do the miles (I don't!) then diesel must be the best choice.
Although I can still justify a petrol for now