Under two dollars is healthy for most offers, and under a dollar fifty is where you want it. Treat it as a directional signal rather than a hard gate, because reporting on it is inconsistent across accounts.
What makes it useful is that it measures the ad doing its actual job, which is moving someone off facebook and onto your page. It fires when the page loads, so it excludes the clicks that never arrived anywhere, which is why it is more trustworthy than click-through rate or cost per click. Someone can click to expand your copy or open the comments without ever intending to visit your site.
The number varies enough by industry that a single benchmark misleads. Lead generation accounts running at five dollars can be perfectly healthy, and a competitive niche can support more than that. What matters more than the absolute figure is the gap between adsets running to the same page. A large gap points at a setup problem rather than a creative problem, and is worth auditing before you touch the ads.
A very low number is not automatically good news either. Cheap landing page views with no downstream conversions usually mean the ad is pulling people who were never going to buy, so read it alongside what happens after the click rather than on its own.