300Yen for 4 takoyaki and 600Yen for 8 sounds sensible. (100Yen ~ 1$ ~ 1€)
Also 500Yen for a 10min fortune telling session is also quite good.
Why isn't it successful? Bad karma? Can't he cook?
His stand could be located somewhere not very compatible to Takoyaki. Like some high-class district, where people would rather spend a fortune for high-grade sushi, than pay a few bucks for a snack.
His stand could be open when nobody's around. Like when everybody's working. Especially fatal if his stand is in some residential district, since not many people can afford to return home for lunch.