The reason this is sometimes expensive is because by removing the condenser the refrigerant gas will escape. The garage will suction the gas out and store it, and then put it back in afterwards. There are sometimes high pressures involved and the gas is nasty stuff. This is sometimes referred to as "charging" and "decharging" the AC system.
Not sure if that's a reasonable price however. I had a brief look and for the part alone you're looking at £60-90. Probably 2 hours labor and gas refill, check for leaks and some sort of guarantee for £325.