As babu said,
chance for flash sale are very less.
But there is chance for daily deal and the community choice deals.
If CS:GO gets discounted, then all versions of CS will be discounted. So if u see the option to vote for CS:GO or CS:Complete in community choice, vote for it. If it get selected then u will be lucky.
If u think its worth it buy it for 50% itself.
If not then wait for last day. If u r lucky, u will get at 75%. . If not discounted by then, u have nothing to lose. U cans till buy it for the same 50%.
PayPal is the best for me. I earn online, and spend it on games - as my dad would never like me to spend bucks online for petty games. The games I've bought till now on Steam Summer Sale '13:-
- Sleeping Dogs
- Faster Than Light
- Saint Rows 3
- Left 4 Dead 2
And, let me inform to all the newbs that if you vote on Steam homepage (
) every 8 hours, on every 3 vote you get a trading card! You must have steam beta trading card access for that.