A circular polarizer is a regular linear polarizer with a device (called a quarter wave plate) behind it that turns polarized light into 'unpolarized' light again (not really unpolarized, but the plate gives the polarisation vector a twist, sets it into rotation, and the light will behave like unpolarized light.)
That device is expensive. Hence the extra costs of circular polarizers.
Some metering devices (be it light meters or autofocus sensors) make use of reflecting surfaces, and polarized light will not work properly. And if your camera or meter is among those who are affected, you will need a circular polarizer. If not, a regular linear polarizer will do.

The effect however is exactly the same, since the polarizing bit in both types is the same.