Horror/Mystery Recommendation (hidden gems/good deals)

ARAYA
https://store.steampowered.com/app/466740/ARAYA/
The game may have some crappy voice acting and animations of the characters are not great. But the sound and the setting was awesome! You could litterally feel someone sneak up behind you at a certain point in the game.
Also 6 euro during sale? YOINK!

Dreadout and Dreadout: Keepers of the dark
https://store.steampowered.com/app/269790/DreadOut/ https://store.steampowered.com/app/418950/DreadOut_Keepers_of_The_Dark/
A low budget version of the old but gold fatal frame games, I did love em but they do have their flaws aswell, Overall they where both good and i can't wait for the third game to be released!
Also 90% of during sale? YOINK!

F.E.A.R 1-2
https://store.steampowered.com/app/21090/FEAR/
Not really a horror game per say but a great FPS with horror elements! The first game is one of my all time favorite FPS and the second one is not as good but still has it's own moments. Sadly the DLC's for both are kinda bad and the third game is not good.
75% of for the complete pack wich lands on 13.74euro wich is kinda expensive from what it used to cost but still worth it!

Little nightmares
https://store.steampowered.com/app/424840/Little_Nightmares/
Not really a horror game but great setting and story! Loved it more than i thought i would
60% of atm for both standard and complete.
DLC's are not as great but still really good IMO.

Might have forggoten some games but i'll check back later.

Darkwood
https://store.steampowered.com/app/274520/Darkwood/

It's 50% off on Steam and HB. If you have the HB 10% additional, you beat the historical lowest price.

First Winter is an enjoyable, short (about an hour or less), atmospheric pixel art horror. I wouldn't have your expectations insanely high, it's not terrifying or anything but it was a nice play-in-one-sitting kinda game. :)

I just realized Favslist has a list of "Favorite Horror Games", it should give you some good ideas. You could check their global list, and mine is here:

1. System Shock
2. Dead Space
3. Realms of the Haunting
4. Unavowed
5. BioShock
6. System Shock 2
7. Gabriel Knight: Sins of the Fathers
8. Dead Space 2
9. Doom (2016)
10. Aliens versus Predator (1999)
11. BioShock 2
12. Fahrenheit
13. Resident Evil 4
14. Doom
15. Sang-Froid: Tales of Werewolves
16. Doom II: Hell on Earth
17. F.E.A.R.
18. Doom 3
19. Distraint
20. Enigmatis 2: The Mists of Ravenwood

Cheers! :D

Mystery - "Open Sorcery" (£0.40)
A text-based game where you play an elemental fire spirit who has been bound by someone to act as a kind of metaphysical firewall to ward against supernatural intrusion. You are governed by an operating system and limited by your knowledge of the world and aspects of your will, and are to protect a handful of physical areas by detecting and dealing with unwanted entities and phenomenon. It's short but long enough to hit a sweet spot for one solid session, and isn't as linear as it may first seem. Strangely engrossing for a primarily text driven game, and has the low-key spookiness that gradually turns up over time. I'd recommend it even at full price given how cheap and original it is, but on a sale this cheap, and with cards, you can't really go wrong.

Horror (humour) - "Cleansuit" (£1.60 if I remember rightly)
Oh hey there's a knock at your door. Oh hey it looks like a serial killer is stood outside your front door, and it's only a matter of time before they get you. What do you do? It's a text adventure that is short but replayable, with a handful of ways to deal with the bad guy, and a good number of ways to die. It's funny in a dumb slapstick kind of way, Cheap fun, worth it for a laugh.

Oh, and of course, anybody who hasn't played "The Cat Lady" should buy it no matter how good the sale is. That's a classic that doesn't use startle scares and actually builds up nicely. The introduction area usually puts a lot of people off but if you can get past it (and meet the queen of maggots) then that's where the game really starts, and you're in for a ride.

Condemned fulfils your criteria. Decent game for the most part, ending is a bit pants though.

Deadly Premonition is horror, mystery and detectiving all in one bundle, and ridiculously cheap. It is pretty much Twin Peaks - The Game, the writing, the characters and the atmosphere are all top notch.
Unfortunately, the price is quite reasonable, as the game is optimized like goddamn garbage and it is pretty much up to luck whether you can run it at all, even after applying all the fixes. If you manage to avoid all the crashes, however, it's a ride.

Unforgiving - A northern Hymn is a nice horror game. Often on sale, definetely deserves some more sold copies. Is however a bit on the short side for playtime. Up to 3 hours if you are just playing through it normally.
But it gives you insight in a lesser used mythology similar to DreadOut which has been suggested earlier already. Those games are nice for the mythology alone.