I've been a cheap bastard with game buying since before Steam was worth using; this has been more due to staying off the upgrade treadmill than anything. Quick markdowns and gold/platinum sets were out there to be had at retail 15 years ago, if you were patient and diligent.
These days, I have a loose tiered system:
Under US$2.00-ish, the game only needs to be slightly intriguing. That's the same price as a large not-fancy coffee, so it's cheap enough to take a stab in the dark. Sometimes I get a stinker (Droplitz, Turba, Words For Evil), but many are decently fun for the price (Little Inferno, Chains, Fractal, Hexcells series, 10000000), and occasionally there's a title that is fantastic for the price paid (One Finger Death Punch, Waking Mars).
Up to US$5.00, the game needs to either be in my wheelhouse of genres (Titan Quest Gold, SupCom Gold), doing something fairly clever or special on its own (FTL), or has gathered a lot of critical praise (Bastion, To The Moon). Maybe the occasional nostalgia buy, if it's something I didn't get around to when it was new.
Up to US$10.00 looks a lot like the previous bracket, but has more than one thing going for it. Dungeon of the Endless was a clever mash-up of roguelike elements & tower defense, and well received critically.
Anything north of of US$10.00 must be something extraordinary indeed, or a hella good all-the-things bundle. Civ 5 Complete has been the only thing I've bought in many many moons that clears this bar.