cmclout: I've been a member of GOG for a little over a year, and their sales seem to be kind of off-the-wall with regard to consistency in sale prices. A game may be on sale for 75% off during one sale and then only 50% off the next time its on sale. For example, I believe I've seen games being sold in this Summer Sale at price points higher than what I paid during normal sales within the past year. I would expect these seasonal mega-sales to have the lowest price points of any sale, but that is not always the case. As another example, a base game (for example, Little Nightmares) may be on sale for 60% off, but the game's DLCs might be only 10% off.
Regarding the game you mentioned, Planescape: Torment: Enhanced Edition, GOG currently has it on sale at 50% ($9.99) and Steam has it on sale at 80% off ($3.99). The GOG version includes 2 HD wallpapers, 16 avatars, 104 artworks, Chris Avellone and Colin McComb book, and the unaltered original version of the game. I don't see any of those listed on the Steam product page. Many older games on GOG have those types of goodies bundled with the game, whereas Steam usually does not include them. Some people would see those goodies as a reason to pay a higher price on GOG. Whether or not you feel that way is, of course, your personal opinion and personal preference.
Regarding Jazz Jackrabbit, while that is an old game, Epic finally decided to release it on GOG last year, so it's still a relatively new release (of an old game). I wasn't happy about the price when they released it, but I still bought it upon release because I remember having fun with it back in the day (1994). I made the decision that, to me, it was worth the asking price, so I purchased it. I'm sure many other people did not think it was worth such a high price and did not purchase it.
One thing we don't know (or, at least, I don't know) is what is involved in determining the price (for example, who sets the price -- GOG, developers, or a mutual agreement). We also don't know who decides when a game goes on sale or who determines what the sale price will be. If the developer has significant input regarding price, which I suspect they do (since that's their income as well as GOG's), then the blame for higher-than-expected sale prices does not lie exclusively with GOG.
You make some good points.
However, regarding "blame", I'm not really blaming GOG. It's just an observation of what has been happening in the past year to year and a half. It's interesting that you mention "goodies". This is a part of what I'm talking about. Even the goodies have changed. They are now asking you to pay for soundtracks. The PTEE soundtrack is now on sale on GOG for more than the game and the soundtrack combined on Steam. I think it's commendable for GOG to include goodies but they are of very limited value in most cases and GOG seems to be moving away from this now anyway. A lot of the stuff they include is available for free.