It's not exactly hidden. It should be staring you in the face on the normal steam store Payday 2 webpage below.

http://store.steampowered.com/app/218620/

Lol. The usual steam sale price but it's a decent game with good player base numbers still. Very little DLC for this game is actually worth the money though.

one of the best games ever.

also payday the heist for £2.75
picked up payday 2 on humble bundle for £1 but if you have a good gaming rig this is a good price.

When is it not this price. Cold

it's free to play for the next 10 days, would suggest you try before you buy. if you like it I would get the goty edition which includes more dlc.