It all depends on the value of the car & it's emissions.
What happens is that you pay tax on the RRP of the car, what you pay is determined by what emissions the car has, so for instance you went for a new Golf S 1.9TDI 5 door, you would pay ~£50 per month, but if you went to an equivalent Pug for instance, because it is a cheaper car, the tax would be less, same goes if you went for an Audi, it would be more.
All 4x4's are taxed very heavily, as are petrols, so it is wise to go for oil burners
Then there is the fuel to be taken into account, if you pay for the fuel then claim the company miles back, you don't get taxed for using it for personal use, as you are already paying for fuel,
BUT if your company pays for the fuel and you have to declare what personal mileage you do, you have to pay tax on that, but depending on fuel prices and
how much mileage you do, it can work out cheaper, plus you can always adjust the figures to suit accordingly.
Also remember that any optional extra's like Sat-Nav and leather seats all add to the price of the car, therefore add to how much P11d tax you pay.
Check out each manufacturers websites, as they usually have company car tax calculators built into them somewhere.
Hope that helps!