Forgive me if I'm overstepping any boundaries in what I have to say. I sell, trade, buy, and observe. I'd say overall the prices across the board are fair, and reflect the PC's trends as well as population numbers, but there's just a few outliers that strike me as odd. These few just need minor adjustments, nothing drastic like dropping half or a third from its current price.
Gelerts have been mentioned (definitely agree the Darigan and Grey should flip-flop prices), but there's also the Darigan Techo which seems a bit on the higher-end.
The plushies are difficult because they're either commonplace or nearly extinct, and this factor conflicts with popularity as well. There's not a lot of Plush Quiggles, because they weren't popular back then when they could be created. But then you have the Plush Gnorbu, a new and fairly popular pet which had a very short window to create before conversion happened. So determining those values, I don't know what to say unfortunately.
I don't know how much anyone wants to align with the tier guide, I personally feel there isn't a need to strictly follow it. Not everyone trades on PC, plenty of people are wanting their pet to purposely avoid that wretched place.
Sorry again if I've no business to speak about these sorts of things or to drop my 2 cents!