I bought my truck in June 2015, and in my area Ram and Ford were priced pretty close while the GM trucks were a few thousand more when looking at an apples to apples comparison. I think this can tend to vary depending on the promotions at any given time and by region.
In terms of resale, I don't think a Ram with a Cummins in it takes a back seat to either of the other diesel trucks. Again, at least in my part of the country. I actually think the diesel Fords tend to run a little cheaper used compared to the Dmax or Cummins, again in my part of the country. Maybe the story is different elsewhere.