First, it's impossible to know what airfares will be in a few years. A lot of the ticket price depends on the price of oil. It might still be relatively cheap then, or if there is another scare or shooting war, the price could jump way up with fuel surcharges.
Delta has a direct non-stop from Detroit to Tokyo Narita, but as far as I know, they are the only ones.
You can get cheaper fares on other carriers, but that likely means making flight connections somewhere in between. Other Asian carriers like some Chinese airlines can give some very cheap fares, but often mean a very long connection in Beijing, Shanghai, or some other city.
Probably it would make more sense to fly from your nearest gateway city like Chicago if you don't want to pay the Delta fare.
You also didn't mention what season you're flying. Peak travel times are the worst, but spring and autumn are the best times to see Japan and fares are not too bad. In summer the fares go up a lot and the weather in Japan gets horribly muggy and humid (unless you go way up north). The dead of winter is usually cheapest but not the best time to see the country.
The best time to start looking for tickets is about 3-4 months before your departure date (unless you have to fly during a peak time). Golden Week, the end/start of the year, and Obon are often more difficult to get a seat and should be avoided.
You can see some fares right now on http://matrix.itasoftware.com