As a part of Devolver Weekend Sale Serious Sam Series is 90 % off, Talos Principle and it's DLCs are 66% off and Road to Gehenna is 25% off.
If you already have Talos, you can upgrade yourself with Road to Gehenna.
Talos Soundtrack and Serious DLC are also on sale so now it's the perfect time to get them both.
http://store.steampowered.com/search/?snr=1_4_4__12&term=the+talos+principle