The two common measurements of a games cost are polygon count and vertex count. Furthermore polygon is interchangeable with triangle in these measurements, as GPUs only see vertices and triangles, not 4 sided polygons. a game character may stretch anywhere from 200-300 triangles, to 40,000 triangles. a high-end third person console or PC game may use many vertices or triangles per character, and an iOS tower defense game might use very few per character.
http://pixelperfectgaming.com/wp-content/gallery/ps4-poly/screen-shot-2013-02-20-at-7-17-56-pm.png
File Size
File size for 3D modelling is known to be vital, without file size you would not be able to create or save your project to begin with. Furthermore, considering having a high polygon count in your 3D environment would make this even more difficult. Also, another valuable thing to have when creating 3D assets is a good CPU that would have you render your work fast and efficiently, although good CPU's come high in price.
http://www.computerhope.com/jargon/f/file-size.jpg
Rendering Time
Rendering is the final process of creating the actual 2D image or animation from the prepared scene. This can be compared to taking a photo or filming the scene after the setup is finished in real life. Several different, and often specialized, rendering methods have been developed. Rendering is also the most technically complex aspect of 3D production, but it can actually be understood quite easily in the context of an analogy, much like as film photographer must develop and print his photos before they can be displayed, Furthermore, computer graphics professionals are burdened a similar necessity.
http://pixelpainter.com/PVC/8-12-12/RenderingTime.jpg
