It's because of people like you that I even bother involving myself in these discussions. You are extremely overestimating their profit, and severly underestimating the development+extra costs.. "Big profits"? They've only sold 560k. Even if this was a cheaper DS titles their wouldn't be as big of profits as you are making it seem. This was easily one of the biggest budget games on DS. Considering Take-Two would only get about $10 per copy sold, they haven't made much money yet.
im not really going into any outlandish exageration, this game certainly was done faster and MUCH cheaply than PS2, 360 or psp games, and the credits do show that the staff wasnt gigantic.Development costs for the most expensive DS game wouldnt even match the average PS2 much less a current Wii game like No more heroes , which granted a sequel after 200k units(and it took longer to get there too). Plenty of DS games show better visual and still struggled to get half the sales of this game, while still warranting sequels. By no stretch of imagination i can be dense enough to figure that at 500k this game is barely meeting ends, specially when last year THQ explained that for certain WII (Not even DS) games, 200k was enough to break even. Its pretty clear that this game will clear 1 million within a small time frame. 1 mil times 10, adusting to your scale, means 10 million in profits, which (and this might surprise you), its considered quite good for the gaming industry,even for games twice as expensive to make as this one.
Not only that, this game lacks voice acting, musical score fees(extensively used in other GTA games), CGI custscenes, complex 3d models and all the bulk of the expenses involved most GTA games.
If you think they arent really making much money , then loads of developers of DS are making much much less, and still making sequels, probably they are doing it for free?.Or i might take my facts, add the fact the developers said they are VERY satisfied and conclude this is doing really fine.