Never mind that we don't know how much a Nintendo 3DS game costs to buy, developers need to develop the games first. And that costs money.
According to Harvest Moon publisher Marvelous Entertainment, Nintendo 3DS games cost — depending on the title — between ¥70,000,000 (US$838,976) at minimum and ¥100,005,000 ($1,198,598) at max. These, of course, are projections Marvelous made, and actually 3DS development costs are expected to vary by title and studio.
Earlier this year, Marvelous Entertainment stated it would not be developing original games for the time being, but is focusing on established series.