Why spend money on building? Barbarians are already great, even without money. They take what they need from others. I play on random maps at 200%, with a patch that increases starting gold to 2500.
I recruit Gurnisson with a ballista. If the heroes have orcs in their squad, it's even easier. We protect them with goblins placed one by one and start shooting. The first and second levels of neutral creatures die with almost no losses. Then, either we add orcs (spending 1000 gold is like recovering from just one looted chest), or we add birds. If there are computer-controlled factions, we capture their Castle, and that's how we get more money. It's good to build monsters, but it's not always possible in the first week. But birds and a Stronghold (or preferably a Castle) are ready by day 7.
As soon as there's enough money for a second small army, it appears and also starts earning money.
It's like this: the more money you spend on the army, the more powerful objects it will conquer, and the more money you'll have in the end.
It's good to accumulate strong monsters in the Castle – not only monsters but also birds and ogres. If you're lucky and the map has angels in conservatories and wyverns in Hives, that's even better.
And then, the utopias of dragons crack like cans of sardines under the mighty paws of gray behemoths. This happens around week 4, but if you're lucky, you can do it in week 3.