Buy cheap, sell expensive - that's the principle of all business transactions. Doesn't matter if it's merchandise or labor.
If it's ethical depends on the context. If you can ask for $50 because you have a reputation of being a professional and reliable worker and then you hire some stranger to do the work without your supervision, only coming later to collect the money - then it's unethical because the owner can reasonably expect that you will be the one doing the job. If there are no such conditions and the owner just expects the job to be done, I think it is ethical and you are being paid for being an intermediary and organizing the work.
You should also take full responsibility if your subcontractor botches the work, and you should repair or pay for damages in such case.