You should not have to purchase the module. Depending on the version of SL that you are using, you were issued license for FRx or Mangement Reporter. If you log into Customer Source and pull up your registration keys you should see some for one of those modules.
The reason that SL does not have pre-built versions of the trial balance or statement of income is that everyone's chart of accounts can be different and may not even flow in the order that you would want the statement of income to come out.