FUNDAMENTALS OF GAME DESIGN, SECOND EDITION
The Construction Converter
Construction is a conversion activity that changes labor, money, and materials into buildings (or whatever the CMS constructs). Construction happens only when the player wants it to, so she controls the spending, and she can usually defer
construction until such time as she can afford it. Although it drains money, the player can see immediately what she gets in return, and in any case, it's an investment because the constructed object does something useful in the economy.