It might depend on the country...when we were SP in the Netherlands I worked about 8 hours a week otherwise we would have starved, most SP did the same. I knew a Bethelite who had a running business , never talked about handing over money.
In missionary-service we saved up our 28 days (missionaries have always an extra 14 days) to go back and do night-shifts to buy a car or so. Nobody ever asked or commented about it.