Upgrade availability and price is always variable, based on criteria that likely nobody outside of United knows - especially for a flight that is still 9 months away. This seems to be doubly true for award flights (see my example above where it was over $5k for an upgrade to PP, and only $100 more for Polaris - which came down to $300 at checkin!).

If you want to 'upgrade' now, your best option is likely check how much it'll code to book the flight directly in Premium Plus. I'm seeing as low as 50K miles for that flight in October, which is only 20K more than what you've said you've already paid. Depending on how you value miles, that's likely less than you'll ever get for the buy-up. Otherwise there's a good chance you'll need to wait until check-in to get a good price - but obviously by then there's a real chance PP will be sold out, and/or close enough to that such that the upgrade prices aren't cheap.