I think it's hard to use the KLR 650 price as a comparison since these are all basically custom conversions for the Marines. From what little I've read, single-cylinder diesel motorcycles are common in China so it's not like they don't already exist in large numbers out there. Then again, until fairly recently, the vast majority of Chinese were perfectly happy with their bicycles, so obviously, tastes and needs vary across countries.

Actually, doing a bit more reading just now, I was rather surprised at how low the horsepower is on diesel motorcycle engines. Very broadly speaking, you're talking 400-500cc of displacement to get just 10 hp. That's not going to go very fast at all.