I have never paid CYC prices using my Polar card at a CYC. I always pay the Polar price.
Well, I can't speak from personal experience as I'm not a PolarPlus subscriber, but I encounter many people grumbling on forums that they've been overcharged and Polar customer support tell them they should have looked at the online map for the prices.
Here's an example CYC point as shown on the PolarPlus map with non-Polar pricing.
Quite a lot of CYC points are free, so the PolarPlus price or the CYC price would be the same; possibly there are some where the PolarPlus price and the CYC price are different.
Not to be confused of course with the PolarPlus vs PolarInstant prices which are different again (and PolarInstant not usable at CYC sites).