I took the liberty of buying the Horror Music Pack, and can confirm the quality is very good. Some great atmosphere backdrops, some chilling themes and a few great action sequences for fight scenes. I suggest you practise with some sample MP3 files in your projects first, as the music is provied in MP3 only. You might need permission from the author to convert them to WAV or OGG for use in FPS Creator

I cannot stress enough the importance of good music and sound effects to polish your creations with. To commission this music professionally, you would pay anything from a few hundred to several thousand pounds. If you are writing a horror / thriller style game, it's worth £12 just to find out if there is anything you can use from the collection!
Just for clarification, my opinion here is that of a fellow game author, not as a TGC bod. TGC does not endorse third party products we do not sell. As our newsletter states, check carefully before parting with your cash!