How much bigger does a precomputed lookup table get when salt is added?
Looks right to me, assuming you know what the 10 salts are. 

If you don't know a priori what the salts are (which is typically the case when building a rainbow table), you'd have to compute for the entire salt space. This is where the big gain comes into play as for each additional salt bit, the storage requirement doubles. Thus you get exponential growth of the storage requirement.