walter - like everything else, price varies from manufacturer to manufacturer - but in this case you dont always get what you pay for. Make sure that whatever unit you buy will handle the total grains of hardness of your water as well as the water usage - i.e. single vs twin tank, etc. Stay away from the generic one size treats all units.
i dont know where you are located in the us so cant refer you to anyone in particular. There is also a new softener out which does not use salt and no drain line is needed, a little pricey but less than some of the quotes you've received.