(quote)... why would a more heavy duty tire be 100 less??????  

Made in China, Taiwan, Vietnam, Cambodia, etc...???

Along with what wyndancer said, most of the 17.5 tires on the market here in the USA are imported from a foreign land. Just be sure you're comparing a top of the line load range H,  17.5 to the load range G, G614....

The Goodyear trailer tire in 215/75/17.5  Load Range H retails for $366.38

The 16" G rated tire is a strange size. There is not much competition in that size and weight range. Hence good year can hold their price point.

Once you move into the 17.5" wheel size your choices increase.  This moves you into a commercial truck tire and there are many manufactures competing.  And while Goodyear still has almost the same list price for it's H range 17.5" tire as it's G range 16" There are many less recognized brands of tire selling for less money. I'm seeing Hurcules, Hankook, Michelin, Goodyear, Brigstone, Continental and many other brands competiting for this commercial truck tire. I see the Hurcules and Hankook brands selling for under $200 but most of the more recognizable brands are over $300 per tire. At least on their list price.

Be aware that there are a few difference when you change to the 17.5" world.  Most of these H rated tires run at 125psi. So you need rims rated for the higher tire pressure. Most 16" wheels are Stud Centered and many use 1/2" stud. Most 17.5" wheels are Hub Centered and we see a lot more of the 9/16" studs.  You often have to change out the axle hubs as well as changing the tire and wheel when you step up.  So if you decide to go 17.5" tires. Make sure the wheels you buy will work with the axle hubs you have or be prepared to buy new hubs at the same time.  SouthWest Wheels offers an Alcoa wheel specially bored to fit 7000 lbs axles that use the 1/2" or 9/16" tapered stud holes for mounting.