The Witcher 3 for $23, Elite: Dangerous for under $26, DiRT Rally for the price of a pub steak and Wolfenstein: The New Order for the price of an expensive smoothie. That's part of what PSN has to offer right now.
There's no special event or celebration for the discounts, beyond everything on offer being under $30. There's plenty of quality available for under $10 and $20 as well, including The New Order and the remastered edition of Divinity: Original Sin.
Here's some of the better deals on offer. Note that the prices listed below are for PS+ members.
Wolfenstein The New Order: $7.55
Sniper Elite 3 Ultimate Edition: $9.45
The Book of Unwritten Tales 2: $10.45
Valkyria Chronicles Remastered: $11.95
Dragon Ball Xenoverse: $11.95
Dark Souls 2 Scholar of the First Sin: $13.95
Resident Evil 4/5/6: $13.95
Metal Gear Solid V The Definitive Experience: $13.95
Assetto Corsa: $16.45
DmC Devil May Cry Definitive Edition: $17.95
Alien Isolation: $17.95
Deus Ex Mankind Divided: $19.45
Little Nightmares: $19.45
The Witcher 3: $22.95
XCOM 2: $24.55
Elite Dangerous: $25.95
Note that the Resident Evil price is for each game individually, not all of them as a package. There's also some other indies and smaller games scattered on the store. If there's anything prominent that I missed that you think should be included, let me know!