TOO MUCH!
Seriously though, had I known what I do today, I would have avoided a lot of crappy decisions. For example, when I started out, I spent a bunch of cash on awful PS2 games with no replay value simply because I didn't know what a good or bad game was to begin with. With PlayStation 2, I probably spent in the region of $300.
Then, I moved onto PC games and also wasted a ton of cash. Eventually ended up just being given games from friends, which made life easier. I just had a cheap laptop and 2 gaming mice, a RAT3 and a SteelSeries Rival, so that's about $200 where I live. Games was around $100, probably less.
Then, mobile gaming... oh gosh the money I wasted here. I wish I didn't buy all those App Store gift cards because I wasted them on the silliest things, from in-app purchases in those awful Gameloft and Supercell games to bland, pointless games like Shadowgun. Can't say it was all a waste though, titles like Alto's Adventure and Monument Valley were beautiful experiences that I will remember forever as great moments in mobile gaming. I'd say mobile gaming was a good $200.
Then there was Steam, the be-all, end-all. Spent so much on CS:GO items and other Steam games. Probably totaled around $700. Complete waste. Don't buy imaginary items guys, buy experiences.