I've only spent money on a mbile game twice, and that was to get rid of the godawful banner adverts. And even then it was about 99p on an otherwise free game.
DLC wise, though...
Fallout 3 + New Vegas, have all the DLC for both. I got the season pass for Vegas, although 3 was brilliant because I got the discs with two of the dlc on through a DVD & Game rental service I was subscribed to, so technically those didn't cost me anything I wasn't already spending.
Elder Scrolls: Oblivion and Skyrim: All the DLC for both again. This time it was a bit more piecemeal, though.
Borderlands 1 & 2 - All the DLC for 2, as well as the Gaige character pack, skipped out on the other one though. for 1, I think I only picked up a couple of them, but I didn't play that one online, so it wasn't really as important.
As for regretting DLC purchases, probably all of the update packs for CoD Black ops 2 - There was a patch that went out one summer that just completely killed the game for me, as I used to play hardcore S&D almost exclusively with a close team of friends, and it just made the "sprint and spray" classes that where already a blight on the game way too OP to the point that was all anyone was playing.
One of my mates has probably spent a good couple hundred quid on his Star Trek Online account, as well as having paid about £300 for the lifetime subscription before it went .Free2Play.