I don't think there is a place you can get a new n64 for less than $100. The only Nintendo N64 available for less than $100 is the used one. A new game will cost you around $500 and the reason behind this is that they are old games which have been replaced by new ones and finding them is difficult. This means that the stores selling N64 games will take advantage and sell them at a high price.
Im pretty sure they dont sell those anymore in any retail store but if your lucky to find one new for sale by someone online it would most likely cost more than $100 since they are rare in that condition.