Surprised nobody else has posted this as thought it was a good price for the game. A second hand copy on disc for the Xbox 360/PS3 is still fetching an average price of $15-$20.
According to Steam sales history, this is lower then what it has been during previous sales as it hasn't dropped below $4.99US (approx $6.70AU) (http://steamsales.rhekua.com/search.php?term=mortal+Kombat+K…)
Mortal Kombat X is also currently listed for $5.09 (thanks to BTRaynes) for those wanting to play the latest edition in the series and again cheaper than what Steam are currently asking for:
http://www.cdkeys.com/pc/games/mortal-kombat-x-pc-cd-key#pro…
Don't forget you can get both games 5% cheaper if you enter the code generated on their facebook page. For those unfamiliar….code does expires after a few minutes and you MUST be logged in to your CDkeys account in order for the code to work at checkout.
The sequel is only $5. I'd be more inclined to buy that instead because this is 5 years old now, and they delivered a massive patch recently that fixed a lot of issues with the sequel.