For an interesting exercise let's consider the lease rate between a 321 CEO and NEO. It's about an $150,000 difference. Let's also consider that a CEO may burn roughly $500 more in fuel per hour (generally the largest difference is on longer legs).

$150,000/$500 gets you 300 hours of fuel burn to break even, or 10 hours a day of usage.

That's actually pretty typical for most airlines to run their a/c at around 10 hours utilization, although some like Spirit were more in the 12-13 hours prior to Covid and things hitting the fan.

Even more importantly, the largest difference is on long legs so using cheaper CEOs on short legs like RDU-CLT isn't a terrible plan.

The problem in the US now is too much domestic capacity, so no one really needs to acquire even cheap planes save for the ones they're contracted for. But who knows, UA has boatloads of cash and buying some cheap 321's might be in their wheelhouse.