Where I live you get every bit of the price of a diesel back at trade and then some. Freind just sold a 2006 gas 2500 with 160000kms on it for 7k. The diesel 2005 with 260000kms on it sold for 18k. I feel the gas would have got 2k more if it was not smoked in.
I do think the jury may be out on the newer gas though and the values they may fetch in the future. Personally I think the newer gas engines are better then they were in the past, the gap between a 2015 6.4 and a 2015 6.7 in 5 to 7 years may not be as much. There does appear to be a shift. My experiance owning both a 5.7 hemi and now a 6.7 diesel there is no differance in cost of ownership other then the upfront cost ( I do expect/hope based on current market demand in my area) I do not do the work myself on the rigs and follow the recomend service plans on both trucks based on milage.