I'm assuming that you have thoroughly explored the F1 Monaco website: http://www.grand-prix-monaco.com/formula1monaco/grandprix/track.html. If not, you may find lots of your answers there.
Be sure to go to the Monaco Tickets part at http://www.formula1-grand-prix.com/redirticket.php?idgp=15&lang=en. There you can click on the grandstand and find a description and pictures of it as well as what you can see from it. When you click on them, they get quite large, so you see things very clearly. That will show you why the prices vary. There is also a clickable map on the same page that shows the track and where the various grandstands are.
The prices shown are for 2015. $1000 per person may be a conservative estimate for 2016 depending on the location and if they go for practice and qualifying as well as the race.
It was great seeing someone ask about this and someone else respond. My husband is a road racer himself at the club level and an F1 nut. Some kind of racing thing has been a part of our trips to Europe for the past 5 years.
Last year we made a pilgrimage to his favorite track, Spa Francorchamps, for a track tour. And we actually started our trip staying in Zandvoort, within walking distance of the track. He went over there to see some club racing on that former F1 track.
On our first trip over together he went to Brands Hatch as a spectator, drove the Nordschliefe of the Nurburgring in a race prepared rental car and visited the "factory" where his race car was built near Belfast. Two years ago we stumbled on a Go Kart track on a little back road near St. Tropez and he drove it. There he learned that French Go Kart tracks are designed much better than US ones and that the karts themselves have much more power. So if your husband and son like to drive as well as watch, there are lots of opportunities in Europe.
It is roughly 3-4 hours to Genoa from Monaco-Monte-Carlo by train according to the DB Bahn website. Depending on when the race is over, they might be able to get there before it gets too late, spend the night and then go on to Naples the next day, which would take 5-6 hours.
This is the link to the DB Bahn website: http://reiseauskunft.bahn.de/bin/query.exe/en. Although you can't buy tickets through it, you can certainly see the current schedules and times. You will see the train changes and you can click on the intermediate stops. Be sure to click on the map to see the route. The shortest and best way I think is along the coast.
I was puzzled by your question about spending an afternoon/evening in Genoa before taking the train to Naples. With most trains from there to Naples taking 5-6 hours with only one change and running very frequently, I'd spend a slow morning in Genoa and leave there NLT noonish. I've stayed in Genoa twice and although you can find some things to see there, I would want to get to Naples earlier in the day rather than later.
If they spend the night in Monte Carlo after the race, the straight run from there to Naples is about 9-10 hours and is shown to run inland through Milan and Florence with 2 train changes. You can manipulate the route to go along the coast by adding a "stopover" in Livorno. It will take about the same amount of time and require 2-3 train changes.
I know it all sounds very complicated, but once you play with the DB Bahn search, you will find it becomes clearer.
You have lots of time to plan for this trip 2 years from now, but I believe it is never too early to start and doing some of this research ahead will help you avoid most, if not all, of the unpleasant surprises.