Good game for up to $6.84 - Recommendations?

personally i had loads of fun with Serious sam 3 BFE not sure if u can buy the gold edition but being able to play 16 player co-op campaign is much fun :)

Valkyria Chronicles fanboy here too. I knew nothing about it other than the video when I picked it up and its easily one of my fave games in the last few years. A bit like X-Com but thats a really unfair comparison as there is more to it. Xcom is one of my favourite ever games though so depends if you like that sort of thing? If you have an interest in Anime or turnbased games buy it without thinking.

For this sale, I;d go with Rock of Ages
It's a hilarious tower defense+tower offense game, with the twist being that for the offense you get to directly control a single massive ball of stone in third-person behind-the-character view.
Graphics in the style of the animated Monty Python sketches help set the tone as well. And to top it of, single-player campaign is a wonderfully insane "what-if" take on the tale of Sisyphus.

Try "To The Moon"

or :
Bioshock Franchise Pack
The Witcher 1 & 2
Mass Effect Collection
Batman Arkham Origins
Borderlands 1 & 2

The Talos Principle

Prison Architect, it's on sale for 4.19€. I always wanted it but was waiting for a good price so I jumped to buy it. And now I don't sleep anymore as I keep playing it x.X
There's also the Shadowgate games on sale if you're in RPGs. Cyber future awaits you! :)

Hyperdimension Neptunia .ReBirth 1

100hours worth of content ,thats... if its still 70% off tho :x

Here are a few you don't have that I'd recommend at the price they are now. You seem to really like open world games so I suggest going with one of the first three:

Dark Souls - $6.79 (You need a controller of some kind, and make sure to install DSfix before playing)
Dishonored - $4.99
FarCry 3: Blood Dragon - $3.74
Risk of Rain - $3.99

Outside of your budget but for replayability I can't recommend enough:

Spelunky - $7.49
Binding of Isaac: Rebirth - $7.49

The Witcher 2