I don't know if you have already gotten started on your re-model yet. You can expect to pay in the ballpark of $80-$120 per sq foot unless you select very unique high-end finishes that will add to the cost. In that area the price per sq ft that homes have been selling at is almost always higher than what you are paying to add-on so you should receive a good return immediately if you hire a licensed contractor and get the addition fully permitted. Once the construction is completed a good designer can find you great finishes at lower prices than if you went to the vendors on your own.
Also, have you considered going green during the renovation, maybe even adding solar panels to further reduce your monthly expenses and add value to the home or using renewable forms of flooring, cabinetry, and installing low water use and electrical use appliances?
Please call me if you would like to discuss further.
Pinnacle Wealth, llc