Some units like doppelsoldners and Samurais and most of mercenaries are extremely powerful but too expensive to train them. Similarly, building outposts and forts will cost a lot that can’t be afford in most matches. However, simply reducing their cost will lead to overuse in turn.
I would like to make an suggestion according to the principle of increasing marginal costs. For example, the first outpost we build in the game costs 150 woods, and the second costs 200 woods, the third costs 250 woods and so on. Or the cost of the first group of five doppelsoldners initially is 50 foods and 100 coins, and then will go back to the orginal price 75 foods and 125 coins for next units.
From my perspective, gradually raising their cost rather than setting it too expensive or cheap will be beneficial to improve their attendance. I hope my suggestion will be of use.