I would say that the fact is 'saving' is singular and 'savings' is plural. This dictates the verb that follows. You could say A saving of $20 is ... or Savings of $20 are ...
A basement bathroom can be built for about $15,200, Fred estimates. Of that, plumbing is the largest component. These prices are based on costs for a small bath measuring about 6 by 8 feet, with 8-foot ceilings. However, many basements are limited to a 7-foot ceiling. Another potential cost-saving location for a new bathroom is the attic.
1. Edit Answer (for another 5 minutes). Nationwide average code to modify a house to add a bathroom (without pushing out walls or adding foundation) is about $37,000. Some are done on the cheap for about $15,000 for a half-bath, and fancy ones of course for 5/4 bath or fancier can run $50-80,000. Obviously, if you can back it up to another bath for easy in-wall pipe access that will help keep your costs down.