I'm thinking about buying a mat myself so I thought I'd share my research so far. As expected different the price is determined by material, thickness and size. Different mats are appropriate for different types of exercises.
If you want to do some general exercises like sit-ups a 'general' exercise mat will do. The thickness of these mats varies between 1 and 1.5cm and are made of pvc/foam.
If you want to do some pilates, guess what, a pilates mat is what you want. These tend to be 1.5 to 1.9 cm thick and made of Thermal Plastic Elastomer (TPE).
For yoga you'd want a yoga mat. These mats are like pilates mats but thinner, around 0.3/0.6cm. These tend to come in roll-up models, I guess for people who bring them to a yoga class or something.
Then there are heavy-duty mats, the one you find in gyms. These are available in thicknesses up to 5cm (I've seen such mats with a thickness of +30cm, but these are not made for general fitness).
As for the length, this is dependent on what exercises you want to do. For sit-ups you'd want support for at least your tail bone, back, shoulders and head. I can't realy think of an exercise that requires the mat to be as tall as you are.
Be sure to check how to clean the mat (as you eventually will need to do this) and how/where to store the mat. Checking for user reviews (if available) is generally a good idea.
Some mats also have special features such as anti-slip, anti-bacterial, anti-odor, easy to clean, able to roll up the mat, etc. These may effect your choice so I thought I'd mention this.
