If I found a heavy duty box blade for $400, I'd buy it too. However, unless you want to search Craigslist constantly or hang out at farm auctions, the only alternative is to buy one new. As I recall, a standard 72" BB can be had for a bit over a grand from Woods or other quality manufacturers. Heavy duty BBs are at least double that. So while heavy is good, adding weight to a good standard BB is still an economical alternative for non commercial use. And, you can use the extra grand you save to get a grapple or other extra implement to increase overall tractor utility and efficiency.
Edit: Gannon makes the heavy duty Woods BB. I believe they are actually closer to $3000 in the 72 inch >1000lb size. Don't be fooled by the many companies that call their 400-500lb box blades "heavy duty". That is marketing hype. Nothing under 1000lbs, even in a five foot BB, is really heavy duty.