Is this a real number.. or a guess?
What "number" are you referring to?
If it is the ONA nurse salary number, then it is 100% accurate, pulled from their web page.
Nurse agency salary rates for nurses are on the web as well and based on where you fit on the salary grid they're paying about 50% more, so the $75 / hour is a conservative estimate, not a guess.
Billing rates for contractor staff are typically 2 - 3 times the hourly rate an agency pays to their contractors. If you hire a PSW to come into your home help care for someone the billing rate will be $60 - $65 per hour + HST. The PSW is paid about $20 - $22. A nurse paid $75 / hour by the agency would be billed at anywhere from $150 - $225. This is an estimate based on 40 years of contract management in healthcare prior to retirement, not a guess.
The URL link is for an article referencing Agency billing of between $80 - 150 per hour. My example used a nurse at close to the top of the pay scale and a billing rate of at least $150 would be common, more is possible.