To give you the idea how much does the local money cost, a Peasant (Human level 1) costs 10gp, and a Bear (level 3) costs about 250gp.
BTW, many units are similar to HoMMV. Peasants are still simplest units, Skeletons are of two kinds: Archers and Fighters, Vampires still suck lifepoints, Fairies are level-one Elves, Elven Archers hit twice, Dragons are Black, Green, Skeleton and Red ones, etc.
However, overall balance is quite different. Low-level units are just as useful as high-levels, or even more useful. Remember that the size of your stack is limited by "Leadership". That is, you walk away with stack of 70 Peasants OR 5 Bears. So the best units in the game are probably low-level Fairies and Skeleton Archers.
Another difference is that you have to fight with the units you've got on sale. And units availability is half-randomly generated when the game starts. People say Giants are cool FAT unit, but I've got no Giants in nearby Castles, and have to cope with Bears as my Fatties.
So, although there is only one map, if you restart the game you would get a slightly different thing to play.
BTW, who said these's 20 hours gameplay time, he is wrong. Show me a man who can do it in 20 hours, a real Hero Genius
Probably if you know everything about storyline, unit abilities, spells availiable etc, you can do it in 20 hours. But I've read every spoiler I could find on Internet, and am playing for at least twice as much time now. I think I'm only about halfway through.