From spending lots and lots of gold on a flying mount in World of Warcraft, to blowing insane amounts of ISK on a rare ship type in EVE Online, there are all manner of expensive items floating around out there just waiting to be purchased. Some folks enjoy spending game cash on cool armor (or dyes to make armor much cooler than it was) or mounts. Others prefer to hoard gold and spend it on other things that are less common like paying other, better-progressed guilds, for runs through certain dungeons to achieve things like epic weapons or even special items like pets or riding mounts.
Beyond the most obvious in-game items, you now have microtransaction models, enabling you to spend real-world money on your game trinkets. On top of that, we also have the items that come from things like trading card game codes -- the spectral tiger from WoW being a highly-sought-after example.
So what's your most expensive in-game purchase? What item could you just not live without?