This is going to be close, but not exact. First of all, the fare would be from Reno to Milwaukee, as Carson City did not have (and still does not have) an airport with scheduled commercial service.
In my timetable archives, the closest United schedule have to 1968 is from January 1970. Unfortunately, it does not show airfares, which is odd as most timetables from the pre-deregulation era do.
So, using a 1973 UA schedule, the one way coach fare from RNO to DEN was $65, and the one way coach fare from DEN to MKE was $73, for a total one way Y fare of $138. First Class over the same routing (the most direct, with just the single connection in DEN) would have set him back only $41 dollars more, or $179! Round trip fares would be double the above amounts.
Keep in mind that these are 1973 figures, so 1968 would have been a bit less, but not much, since the CAB kept pretty tight control over fares in those days.
Hope this helps.