Some organizations openly set out the first few weeks or the first month or two as a probationary period for new hires, but you're right about the cost effectiveness to some extent. A six month recruiting process might cost a few hours or days every so often from a few employees, but won't add up to even a single month's pay for a bad employee. Add in a cultural practice of risk-averse hiring (don't hire any B or C players) tilts the scales even further.

That said, I can imagine some cases where opportunity cost and being more concerned about getting good employees than immediately weeding out potentially bad employees would lend one to the probationary thing.