At 6PM GMT (5AM AEDT) GetGamesGo are having a 24 hour sale for Serious Sam 3 which is usually priced at $40 on Steam & their own site. (That's 66% off! & When converted to AUD is even cheaper ~$12.75)
You can also get a few of the other games in the series bundled for $3-$4 from the latest Indie Royale bundle - http://www.ozbargain.com.au/node/62293 - Which expires in about 29 hours. (2 hours after the GetGamesGo deal expires)
-SS3 is a Steamworks game, so therefore requires a Steam account to activate.
-GetGamesGo, if you were unaware, are an authorised Steam key reseller.
GetGamesGo also have Deus Ex series games on sale for 48 hours which can be seen here.
Also, the "Seriously Digital Edition" of this game (which includes some DLC, soundtrack etc.) is $16.99: http://www.getgamesgo.com/product/serious-sam-3-bfe-seriousl…
EDIT: The "Seriously Digital Edition" is actually only US$13.59 when put in the basket - possibly because the "standard" edition in the OP seems to be sold out.