Free...and even then, it'd better be easy to impulse "buy", otherwise I won't bother..
Hmmm....impulse buys for me are usually for games I'd like to check out but have no desire to pay full price for. Normally I'd say around $15 to $20. I've also grabbed many copies of games from Steam sales that I already own on console, just because they were only a few bucks.
I'd like to impulse buy a few Wii U games, but Nintendo never seems to drop the prices on their first-party stuff anymore which boggles my mind. There's a ton of people who will only buy certain titles cheaper. If you never drop the price, you're throwing away sales.
Depends for Retail under $20 is an impulse by but only if it from a developer I like it is on sale and the price is good.
For Digital it has to be less than $3 for a single game $5 - $7 for a bundle.
Though they aren't really impulses because they are games I shown interest in but I didn't pull the trigger because money is tight.
Usually the $20 or below area.
Listen to Iced Earth and play Doom
This is clearly something personal, where someone might consider anything remotely interesting priced below the €30 mark to be an instant buy, while others might balk at the thought of spending €30 on a game without giving it much thought. I myself will snag anything I'm remotely interested in if it's no more than €20 if it's a physical release, or no more than €5 if it's digital. I'm assuming that my impulse buy prices will be lower than most of you on here, but truth is it prevents me from buying too many games that will just sit unplayed in my library.