HNL to ORD on 781. Booked PP on miles. $575 offer to F. F cabin prob 2/3 full, so this is prob the best it's going to get.

Flying CPT - EWR on Thu, Mar 16, 2023 and currently in Economy. The cash upgrade price to Polaris Business was $1,382 for a while, but it has been $6,047 for a week or two now. Did I miss my opportunity for a reasonable cash upgrade or could it drop again?

Still J9 inventory, the answer depends on how the remaining seats sell. Anecdotally, I've seen fluctuations but they can also hold steady. Best advice is to keep checking.
Edit: The one-way price is quoted at around $3000 booked into D, have you looked into "changing flight" into business class? Might yield a favorable result.

Recent offers when booking PP:
EWR-LHR ~$1,200
ZHR-EWR ~$2,600

Both flights had PZ inventory available (the flight from Zurich was PZ9.)

Then why not have purchased main cabin if PZ was available?

Because I wanted to be assured of a PP seat in the event of something going awry.

PZ inventory would get you upgraded to J at booking, from economy.

Are you referring to IRROPS? Given PP is the smallest cabin, there may not be enough seats on the next flight. Downgrade would be less likely.

United smiles all the way to the bank.

Purchased an int'l round trip in PP, used Plus Points to waitlist for upgrade to Polaris. Purchased domestic round trip in economy, assumed would get CPU offer at some point.

In both cases i received upgrade offers within 24 hours of booking. In both cases the final price including upgrade offer came to about half what I would have paid if I had purchased Polaris and F to begin with, so I jumped on them. Plus Points for int'l itinerary waitlist upgrade were returned to my account immediately.

$399 for a 2.5 hour flight where you presumably already have E+ when you factor in United's awful catering strikes me as far worse than an okay deal. I wouldn't pay half that for that route.

if you really want it price out rebooking into first. It has been much cheaper for me recently than paying for the upgrade

As a Platinum, you might even eek out a CPU on that flight. I certainly wouldn't pay $350 to upgrade.

MSP to DEN on a 738. $149 Y to F.
Don't think I'm gonna bite, even as a Silver have had luck in the past with CPU on this segment.

TPA to SFO on a B39M. $399 Y to F was the offer after booking. Not bad, about $200 less than just booking F directly. I'll be curious to see what it is at check-in since F doesn't look like it's very booked up.

1/24, LHR to DEN on a 789, booked an award ticket in Economy, and saw offers of $599 for PP, or $699 for Polaris.