Site Plans and Tax Map Merge- AutoCAD

I am needing to update several site plans with an overlay of the tax parcels. One file is a tax map and the other a site plan. We would like to merge the two. We have the files. This will be an ongoing project because we make changes annually. Would you be willing to do a flat fee per property? You will need to have AutoCAD experience. I have about 30-40 tax maps to be updated. You will be provided the existing CAD files.