Brilliant!  Umm.. How much does one of those cost? 

Actually... The traditional design violin already does incorporate a cup holder as an integral part of the design.  You just need to use a rubber band, as shown in the photo..

 Now, back in the old days, they used to make cups that actually fit, and the rubber band wasn't necessary.  Which was a good thing, because rubber bands hadn't been invented yet. 

But then in about 1751, the shop that used to make the cups went out of business, and violinists the world over have had to search for the valuable antique violin cups, or just make do with the best as they could manage.

There are companies that still market cups that (with a rubber band) work for the purpose.  Like the "Solo" company, whose main target market is obviously soloists, and the "Dixecup" company, which markets more to those who play in the Southern rock or country genres.

That part of the violin is called the "C-bout" which is, of course, short for "Cup-bout". 

Experts still argue over whether the part that appears to be another cupholder on the other side was made to hold a second cup or (as some theorize) a small cheese tray so violinists did not become hungry enough during long rehearsals to begin chewing on their own instruments. 

There are some playing holdovers in technique from those times.  Violin instructors still repeatedly remind new violinists to keep their scroll up, because otherwise the contents can spill out and from the audience's viewpoint it would appear that your violin had just piddled on your foot. 

It is not known for certain if violinists in the 1700s just tilted up the scroll enough to be able to pour a sip of their chosen libation into their open mouth, or if they had something like our modern crazy straw that could be used for more clandestine sipping.

Modern players are of course able to use whichever option they are most comfortable with or that their teacher recommends.