No, there is no cost break for  adding an elevator. The Americans with Disabilities Act prevents discrimination in housing and requires certain accommodations upon request (e.g. handicapped access bathrooms, or, as in your case, an elevator) BUT does not require a builder, contractor, landlord (for a rental), etc. to pay any part o the construction cost: the person requesting the accommodation must pay for it. And the ADA also does not provide any funding for the construction: it justs that you have a right to it, assuming you pay for it.