I agree, a lot depends on the map template, but usually, even on the poorest maps, you can gather 150-200 demons by the end of the month, which is quite a lot. That's why diplomacy is perhaps the most important skill for Inferno. If it's allowed, of course.
That's an interesting point about diplomacy – I hadn't considered that strategy. It might not appear as a skill for any of the heroes. Or is the suggestion to level up each purchased hero a little bit at the first opportunity? Just so that at least one hero has diplomacy.
If you really don't have any money, which is rare for Inferno, you can just skip buying them.
For me, having money for Inferno is rare. :)
I haven't played in a while, but recently I tried playing at 200% difficulty against 3-4 AI opponents on M maps. On simple templates. It's quite difficult, I must say; usually, I can't develop well and have to admit defeat. Of the neutral castles nearby that can be captured, there are 1 or 0 (depending on the template), so you can't really gather many demons that way. :)
I remember playing at 160% with save-loading against 7 AI opponents on M, with random templates. There, I was able to gather more demons because it was easier to take castles from the AI (160% after all, not 200).