I have done this trip via boat and on the roads and the difference in time and/or money is negligible. Sitting on french motorways is horrendously tedious but in these covid times, a better option than a boat.
1400 miles each way is 2800 miles. At your 30mpg thats 93 gallons. A gallon is 4.5 litres, even at 1 euro a litre its 418 euros just in fuel, and its 10% more than that ?ish at the pump.
You then have a least one overnight stop each way, somewhere crap is going to be ?50 ish, each way, so thats ?100 minimum. Plus food and drink for 2 days.
Plus the euro tunnel return ticket which is at least ?100, and the french tolls will be a lot more than ?not much?.
So I make it easily ?600-700 round trip if you dont eat en route/or take your own provisions.