As long as the cost is in line with the amount of content you get then DLC is great. Unfortunately it usually isn't. There is absolutely no reason for me to be charged $2 or more for something like new costumes for a character, or $10 for 4 multiplayer maps as Rocket Edge mentioned. I find actual expansions are more often than not priced appropriately, but costumes, levels, and additional characters are priced ridiculously high relative to the cost of a game more often than not.

Honestly, the best way to get companies to stop taking advantage of people with DLC is to just not buy it. Since buying the PS3, the only DLC I paid for was the Metal Gear Solid pack for Little Big Planet and I thought it was worth what I paid. I've never gotten anything else that wasn't free though because most of it isn't worth it. If enough people voted with their dollar then companies would probably take notice.