As for us, we expect a slave who joins our home to work outside the house. It doesn't necessairily have to be a full time job, but they are expected to participate in the household. We also don't require this because we can't afford it -we can very much afford to take care of a slave, it's just part of what we seek. Money they earn would be for their lifestyle, their desires, entertainment, etc.
We have spoken with 2 slaves regarding this, and the male slave was very clear that he had no desire to hold a job or contribute to the household, other than in sexual pleasure. This is not something we were interested in, so we were not interested in him. I think part of our thinking is that we want a stable, responsible person to join us. And if they are someone who can't hold a job or has no desire to work, they will be a rather *lazy* member of this household (for a lack of a better term).
Now ... as all things in D/s, this isn't ALWAYS the case, this is simply our opinion and preference. So if you are in a 24/7 and don't hold an outside job, more power to you. :)
Nyla