PC development hardware basically accounts for 0% of the total dev budget. Its insignificant.
Dev kits, dev software account for a little.
Probably 95% of the 'core' cost (not marketing or anything else) is salaries.
Total salary = number developers x time spent developing (average) + contractor (one-off) costs
You can make a Wii title with less developers (less artists, less programmers), in a shorter time (shorter dev cycle).
Typically, designs are simpler too: its not like you pick a game design, THEN pick the platform - its the other way around. So the Wii gets simpler, cheaper games - which are a LOT cheaper to make.
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This is *development* - not marketing or producer level costs. Note that typically a publisher pays for development (fixed amount), and takes all the 'profit'.
For a $49.95 RRP title, a 3rd-party publisher will make around $17-$20 per unit.
For a $39.95 RRP title, a 3rd-party publisher will make around $13-$15 per unit.
A 1st-party publisher could make an extra $5-$8 per unit (on top of 3rd-party).
(rough figures, they do vary a lot)
PS3/BluRay manufacturing costs (per-unit) are higher than both the 360/Wii (by a couple of $, but this can vary).
Gesta Non Verba
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