The kind of investment I am talking about is effort. By all means, spend money if you want to, but it is not really necessary.
Music is incredibly important in setting the mood. If you have ever seen one of those ‘making of…’ documentaries where they show some classic scenes but without the soundtrack, you will know how much music can impact on how we perceive a scene.
If you set your gaming area up with background music, not so loud as to be a distraction, but loud enough on set the tone for the game, it can have a massive impact.
If you want to go the extra mile, you can have a playlist of different tracks for different scenes. Then you can kick in the scary music when you enter the spooky house.
You can also clear your desk or play area of distractions from the mood, don’t have a credit card bill as a bookmark, or it could be a distraction.
Turning down the lighting is a really simple way to setting the mood.
If you want to throw money at your game then you can get knucklebone d4s, or thematic dice trays and towers. I think proper traditional tarot cards fall into this group as well.
It is also worth putting some extra effort into props and GM handouts. These can be very easy to make or find, it all depends on the setting for your horror game of course.
These tips are taken from my Scarier Horror Play book. You can get 40% off this book at DriveThruRPG using this discount link. You can get 40% off all new releases as a loyalty discount if you join my contact list.