Most of the time the cost of the prototype part lies not in the actual making of the part but the setup for making of the part. In machining terms each time the part is moved, or turned, or taken to another machine it creates a need for a new setup. Each new setup might require new programming, new work-holding, new offsets, and a new operator action like opening the doors and pressing the start button. This all takes time and as such costs money.
The ideal part therefore requires as few setups as possible. Usually most in a 3 axis mill parts require at least two setups, machining the top and the sides ( sides 1,3,4,5,6), and machining the bottom (side 2). If sides 1 and 2 had other features such as holes those could be cut in those 2 setups. As faces 3,4,5,6 are being cut radially introduction of features such as holes or pockets would require a separate setup for each face.
If you as a designer can limit yourself to as few setups as possible the cheaper your part will be.