A PS3 dev kit can be had for $2000, Wii was $1700 when released (likely lower now), 360 is even cheaper for the smaller developer if you can fit the game within XNA, otherwise you will need a full dev kit of which I am completely blank on cost but it should be no more than $2000. So, to develop for al three, it would be $6000 up front, not too bad considering the extra revenue streams.
Of course, that doesn't include some other fees likely to be associated, but it's the startup cost.
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