More valuable - a 90/110 with a Tdi is generally worth more, or at least worth more in parts, than a V8 or 2.5 petrol one.
Same with the RRC - there are dozens of 3.5s and 3.9s out there quite cheap, but try finding a decent 200 Tdi or 300 Tdi RRC at a reasonable price - nigh on impossible!
Just supply and demand methinks