From an SL (partial) tutorial/post:
"For each object in the Second Life world, Second Life compares three important performance factors: download weight, physics weight, and server weight. It then chooses the highest of these weights and assigns it to the mesh as the mesh's prim-equivalent value.
Normally, only the resulting prim-equivalent value is visible in the Viewer. To view physics weight, set ShowAdvancedBuilderOptions to TRUE in the DEBUG SETTINGS window.
Note: The L$10 base cost to upload a mesh model is a temporary rate for the open beta period. This fee may increase after the mesh upload feature leaves open beta.
Here's a very quick overview of the different weights; for more information on each, follow the links below:
Download weight: Calculated by determining how much bandwidth is required to download and view the object. Larger and more visually complex objects have a higher download weight. You can reduce the download weight of complex objects by generating or uploading less complex meshes for differing levels of detail when you upload a model.
Physics weight: Calculated by determining the complexity of the object's physics model. You can reduce the complexity of a mesh's physics model by using the analysis and simplification tools in the Upload Model window, by uploading your own less-detailed physics model, or by choosing a different physics shape type, such as Convex Hull, on the Features tab of the Build Tools window. Vehicles must have a physics weight of 32 or lower, but may have higher download or server weights.
Server weight: Measures the impact an object has on Second Life's server resources. Objects that are composed of many prims and have physics enabled and/or contain scripts tend to have high server weights."
The word "calculate" is used all over mesh tutorial posts. As a failure at mathematics (how I passed physics, I have NO idea), I say screw mesh. Maybe there will be nice things out... cool, yay... different strokes for different folks. Call me stubborn, call me old-fashioned (or just old, lol) but it was WAY easy to learn and begin building with regular prims. Yeah.. no to mesh until they make it easier and more affordable - speaking as someone who lives week to week and likes making her rent.