Estimating plumber cost is next to impossible because of several factors. First, local labor cost vary widely from place to place. Second, the conditions of the job site. It may be as you describe, but there could be other factors that would only be apparent to the professional on site. There are those who thing wall hung toilets are wonderful, and those who wouldn't want them. Good arguments both ways. My personal feelings are for the standard floor mounted toilets because of the many styles available. I am a strong advocate for the Toto brand, but that's a whole story by itself. You may be right about the cost being a wash, but the best way to tell is to have a local plumbing contractor, or better yet two or three, give you a cost estimate based on their inspections. You may be in an area that gives rebates for installing new low-flow toilets, and that is sure worth checking out. Just realize that all low flow toilets are not created equal. Old name brands have lost ground and often are a very poor choice. Local plumbers often will quote you on the brand they sell. Hold out for Toto. They can get them. And, welcome to the forum.