I haven't tried using a secondary hero. And when I used a main hero with only 1 troglodyte, the opponent had over 700 of them.
Protection against tricky opponents =)
And in general, the Grail is strange in their game. What's the point of giving a bonus in the city if you don't give money to buy out that bonus.
In Heroes 3, both the bonus increases and more money is given, so you can buy out what came in excess of the plan.