Its part of the build ability attached to the builder. Basically go to ability data. Then find the build ability your unit uses. Then edit any of the build choice field (will bring up a window). Select your unit from the list. Then edit the parameters for the unit (cost, time, type.
Advantages of SC2...
Multiple units can build same unit with different settings.
Disadvantages...
More difficult to change.
You could have 2 builders for example. One which builds the exact same building fast but at more cost and one which builds the building cheaply but very slowly. This was impossible in WC3 without buggy and complex jass code.
Anyway, check abilities.