A Dover to Calais or Dunkerque ferry is usually the cheapest way to cross the channel.  Dunkerque is in France, east of Calais.  The Eurotunnel is the fastest way, but it will almost always cost more.  I have found that you can usually (subject to availability) turn up for an earlier ferry or Chunnel without having to pay more, because they can give resell your space on the later service, but if you're late they will often charge a penalty of about £40 to change your ticket.

The cheapest prices I have seen are £35 for Dover to Dunkerque, £50 for Dover to Calais, and £100 for the Eurotunnel.  Double these prices for the most popular high season / midday crossings.  There is often a loophole where you can buy two day trip returns for less than the price of a normal ticket, but the ferry companies are wise to this and may disallow travel if they can prove you didn't intend to travel back on the same day etc.  If you get the right ticket, the savings using this method are small anyway. 

Finally - don't forget to take your passport.  Sometimes they don't check passports on the way into France, but they always check on the way back.  You will probably be allowed back in the country (assuming you speak perfect English and have other ID etc), but it's a hassle.

Dover to Dunkerque
Dover to Calais
Harwich to Hook of Holland
Hull to Rotterdam (Holland) / Zeebrugge (Belgium)
Edinburgh to Zeebrugge (Belgium)

Norfolk line is often the cheapest ferry crossing the channel.  It goes from Dover to Dunkerque, which is just east along the coast from Calais.  The crossing takes 2hrs, which is about 1/2 hour longer than Dover to Calais, but it takes about 15 minutes off the drive to the Germany.  It takes about 1.5 hours to drive from London to Dover.  

Dover to Dunkerque at its cheapest costs about £50 in low season, up to £100 in high season.  Book an early or late ferry to get the best deal.  I have never had a problem getting onto an earlier ferry on this service. If you need to change a booking, call 0844 847 5042 (or +44 208 127 8303 if outside the UK) or email doverpax@norfolkline.com 24 hours in advance of your sailing time. I wasn't charged to change the details of the car on a booking in 2010 but you may be charged if you want to change to a more popular ferry time or won't make your original booking time.

You can easily access the motorway from Dunkerque.  Book direct for the cheapest prices:

www.norfolkline.com

The P&O Ferries timetable is a bit of a complex beast, but ferries leave about every 45 minutes in the daytime, and about every hour throughout the night.  The Calais ferry terminal is about a ten minute drive from the duty free supermarkets.

Crossings take about 1.5hrs and the motorway to Germany can be picked up easily.

The Chunnel takes 35 minutes, an hour less than the Calais ferry.  Note that you can't take an LPG car on the Eurotunnel, and it is probably more economical to take a trailer on a ferry than the Chunnel.  You can also load up on duty free on the ferry, but not the tunnel, although better deals are probably available in the supermarkets and booze warehouses around Calais.  

The Eurotunnel operates 24hrs a day, 365 days a year, and leaves every 30 minutes.

The Harwich to Hook of Holland service takes 6.5hrs and leaves twice a day:

The ferry adds about 10 hrs onto your trip compared to driving from Edinburgh to Dover.