Price /Sq Ft of hypoallergenic carpet?

[John Collins]

I have a property that has been on the market for a while. I finally found someone interested however she is worried about her kids allergies with the existing carpet (used upstairs only, downstairs is all wood or tile). I am willing to offer to replace the existing carpet with hypoallergenic carpet - does anyone know how effective that is for people with allergies and what the general cost /sq ft is, with and without install?