Jim,
Weight is by far the best thing you can utilize in your box blade. Too many times I see a light duty "economy" brand box blade purchased and the new owner add blocks, steel, railroad ties, or worse yet, people, to get that box to cut smoothly.
Your estimate for cost is pretty close for the days prior to the import steel tariff. Our domestic steel suppliers jumped on that 30% margin like ducks on a june bug. Bottom line is not only did import steel go up 30% but US steel companies took an additional 30% margin as well. So the steel as a function of cost of sales is now a little more. Most companies are finding it necessary to adjust their pricing for next year by 5 to 12 percent. Of course, box blades are the loss leaders of the 3 point world, kind of like milk and bread at the grocery store. Manufacturers make less on boxes than any other product, but hope to make it up on the rest of their products. So look for price increases at the upper end of the scale there. Many manufacturers were locked in for the year so haven't published their increases yet. Some took a small increase already in hopes prices would go down. If they don't, look for a another small spring increase. Still others don't plan an increase. I suppose they either feel like they were making too much margin or they don't realize their true cost of sales. Maybe the accounting department doesn't talk to sales. A few companies are moving their processing out of the US, since the import tariffs only apply to raw steel, they hope to avoid the tariff that way.
Oops, got off topic there. At any rate, options aside, always get the heaviest box per foot of width you can handle. Not only will it hold up better, you will find it much easier to control the cut and smoothing.