To better understand rapid prototyping services cost, let's get on the same page about some of the elementary truths. In order to make things with robots (3D printers or CNC machines), we first need a 3D file of some sort. For 3D printing, this usually means an STL file. Although we can convert other common file formats, such as STEP files, into STL files for 3D printing free of charge. Basically, we need a 3D shape somehow so that the robot can fabricate it for us.