I use a site called RepairPal. It gives reasonable estimates on everything. I don't own a Ford Flex, but looking up your car (used 2010) this is what it says:
The average cost for a Steering Rack Replacement is between $646 and $1227. Labor costs are estimated between $246 and $311 while parts are priced between $400 and $916. Get a personalized estimate based on your location and specific car. Estimate does not include taxes and fees.
This is a national average.