There isn't a formula. When I built a bunch for books following Run Faster (Data Trails, Chrome Flesh), I built a spreadsheet to help me understand values. The problem is that, the further along in "life" you are, the more variable the value of a life module is. You have to factor in all other possible module choices. In terms of Karma, which is the elementary building brick, a given module that can potentially jack a skill to rating 5-6 is worth more than one that only sees you spread 2-3s over many skills. Plus, there is the soft concept of some skills being more useful than others. Perception and combat skills are more useful that Indusrial Mechanic, even though they cost the same.
So, at the end of the day, I wasn't about to invest the time to build a 7 dimension array of every possible life module. So I took a handful and did this exercises, looking at the "value" of existing modules. Then I crafted the new modules and check them against the same benchmark to ensure they fit the same value (more or less) was achieved, and also eyeballed them to ensure they were equally "useful".
So... this is an art, not a science.