l just would like to know before having it done

Well if u r replacing it yourself which would be the best way to save money- but it takes a long time though- u would expect to pay about $140 to $170 for the alternator itself..but if u rather consider not installing it yourself than labor would probably add another $200-$300 plus tax depending on where u go. As u can see the labor is way more than the part itself. So u will spend under $500 but u will save a lot more money if u get a friend or connections to install for a bargain. Here is a site that has the price: http://autopartswarehouse.com/shop_parts/alternator/infiniti/g20.html

Alternator $445
labor-$200