i have 2 types of contracts: exclusives, non-exclusives... exclusives will never see the result release to public or sold... non-exclusive, the client have no control whatsoever on what i do with the result...
exclusive is always between 5 and 20 times the price of the non-exclusive, depending on the possibility to find new buyers for the script once completed.
as an example, right now i have a 8000$ job to code in the next 3 months... will never be released... would have been a 500$ job if i could sell it afterward, and i'm sure i would have tons of clients... but the guy want it to be really exclusive...