Do more expensive bib shorts make a difference when sat on a turbo trainer for hours on end? Several times during the year I occasionally have to work night shifts (mostly during winter). This can make longer training sessions difficult as the shifts which allow this type of training ride (+2 hours) that are not days off finish at 1:00am. Going out training at this time in the morning can be fun but I more often than not hit the rollers (deer, drunks and potholes). I would like to get in longer rides this coming winter but am finding sitting on rollers for more than one hour and a half painful. Would a 'top of the range' pair of bib shorts designed for long distance (assos s7) riding help ease the pain?
The big difference between rollers and the outside is that your position is much more static inside. If you are good enough on rollers to stand every once in a while that might help. ( I'm not ). Maybe just take a 5 minute break every hour. 

If your shorts/saddle are comfortable outside for 2+ hrs, I doubt expensive shorts will really make much of a difference. However, when it comes to saddle comfort everything works for somebody and nothing works for everybody. 

I'd try the expensive Assos cream to start with, it's only a $30 experiment vs a $300 one.