Correct -- you only need to add $50/hr. for the instructor. It's still quite a bit less expensive than what you're proposing. Care to share the general geographic area in which you're located?
Are you "trapped" into using a Navajo for this role? If your business concept is still in the initial stages and you're exploring different twins for the role, a smaller twin would seem to be a better fit so you'll be able to compete in the local market. Or are you really wanting a Navajo for personal use and trying to justify it somehow? It's one thing to try to figure out a way to monetize a Navajo because it's all you have to use, quite another to intentionally select it for the training role. There's a reason you basically don't see this being done anywhere else.
There's not a huge demand for ME training for the most part -- and those that want it, usually want it for the least training dollar investment possible. You're seeing more and more of these block package offerings out there nowadays. You're also competing against academy-style 141 schools which offer integrated multi-engine training packages. The market tends to get slimmed down somewhat by the time you see the applicant who would be interested in your services. They don't want to go the academy route, they don't want to buy a package, they just want to work with a local instructor and plane. At that point the price-shopping comes into play and that's where you'd be at a disadvantage operating a Chieftain.
When I was instructing full-time, most of my students were multi-commercial add-ons, with only the occasional ATP or time-builder. That was in one of the busiest training markets in the U.S. Pilots on a career track don't tend to have a lot of disposable income.
I welcome any and all input. I am trying to sort everything out, and establish the extent of this niche market. I am not "trapped" into this. As I will retire from full-time professional flying in few years, I decided I want to keep on flying, part time, spanning about four days, 24 hours, with maximum of 8 hours of flight time per week. Little bit of personal flying, teach some, some ground school, work on the airplane, create a little school with unique aero-club like ambiance in a hangar.
So I looked at the Twin Comanche, Baron, Seminole, Twin Velocity, Seneca, Aztec, Crusader, Cougar, Geronimo, Skymaster, and the Navajo. I studied the performance of all these airplanes. I would be pretty happy with say a Seneca under normal circumstances. Problem with most of these light twins is when you loose one engine. The single engine performance in a Seneca I on a standard day is weak 190 fpm. Crank the temperature up to 95 and the Seneca's already weak single engine climb now becomes hang-on for dear life 90 fpm, practically no climb.
The normally aspirated Navajo will climb at about 320 fpm on a standard day, and will still climb at about 230 fpm on a 95 degree day. Furthermore, Seneca I with 93 gallons of fuel, a student, instructor, observer, and few flight bags will be pretty close to it's maximum gross take-off weight of 4200 lbs. The N/A Navajo with the same 93 gallon fuel load - good for a 2.5 hr. training flight - with the same student, instructor, observer, and some flight bags will be 1200-1400 lbs below it's maximum take-off weight, where it's single engine climb will be around 600(!) fpm. This bird will get you out of a jam! That's guaranteed safety my friends. Health and life have no price tags to them.