Ferries to France form a very large and highly competitive market. In spite of there being many extremely cheap flights connecting the UK to France and elsewhere on the continent, they are no use to someone who wants to take a car. Also, taking a ferry is often a much more pleasant experience than spending time waiting and queuing in airports and going through oppressive security checks.
There are many different ferry links between UK and France. The most popular routes are Dover to Calais, Newhaven to Dieppe and Dover to Dunkirk, amongst others. NorfolkLine, SeaFrance and P&O Ferries are the most common companies running regular connections.
Once you have decided which route is most convenient for you, you can then start comparing prices for various times of the year. There are various sites such as directferries.co.uk and ferries-to-france.co.uk which can help you. Searching for your dates, times and routes will provide you with all of the possible results, allowing you to compare prices.
Crossing the Channel by ferry does not take long. Dover to Calais is about 90 minutes while Dover to Dunkirk is around two hours, regardless of the ferry you take.
Prices vary considerably, but they are always much cheaper than the Euro Tunnel alternative. A return trip with a small car should cost upwards of £50 with NorfolkLine, which is often the cheapest company. The most you should expect to pay should not be a great deal more than £70, unless you have a particularly big car.