Paul, I think there is indeed a specific psychological phenomenon where people assume a lower-priced thing is less valuable than a higher-priced thing, within certain parameters. (Typically this applies to customer beliefs about the quality of physical goods, but it probably could apply to services as well.) I don't know if I'd assume, necessarily, that that's what's going on with PIE being only $3 vs. $10. Like, to the average person who has no idea how running an interactive website really works, $3 might look cheap or it might look expensive, $10 might still look like a bargain or it might look like a total ripoff. There's not much to judge it against; other forums have very different systems and typically just say "donate as much or as little as you want, or nothing at all".
If the PIE thing has been working for ten years, then I'd say it's proven itself and that's at least one good reason to keep at it and adjust prices as needed.
Just be aware, raising prices might hint at higher value and attract more PIE buyers--or it might price more people away from buying PIE. Which outcome is more likely is a judgment call you'd have to make based on the demographics and psychology of Permie's website users, which maybe you either know well or have privileged ways of finding out.
I'll just say anecdotally, for me, I found "PIE" very confusing. Right off the bat, it's rendered in all caps, which makes it look like it's an acronym for something (this impression is easy to get on a site laden with esoteric acronyms--SKIP, PEP, GAMCOD, etc. etc.--to newcomers these acronyms act like a pile of shibboleths), implying a more complicated concept rather than simply "donations you can make to help offset server costs". In fact I am still not sure whether it is or isn't an acronym, but "donations you can make to help offset server costs" is so simple and familiar to web users in general I think it would be unwise to muddle it more than necessary.
But also, the benefits of PIE are rather complicated and unusual also, especially once you get into transferring it to other users, and the fact that it unlocks usability features that on any other forum would be standard. Like, as an experiment I wonder if you'd actually get more donations-to-help-offset-server-costs if you simply had a button that said "Donate a few bucks to help keep the site running", and donating unlocked just one or two simple but valuable features, instead of trying to motivate us with sticks (bad usability, limited content/features) and carrots (good usability, little icons, additional content/features, etc.). For all I know you've tried this already and found it doesn't work as well as the current system, but it's just an idea I had. I dunno. Just trying to be helpful.