hmm ok - if you want to get technical - extraction fans are measured by how many cubic feet (or whatever) they vent in a certain time.You have to measure up your grow room and go from there - although - I would rate airflow more than "new air".
Unless your room is sealed, "new air" will probably take care of itself. Just open the door/lid for an hour or so once a day and it should be fine. Moving air will strengthen your stalks and stop CO2 accumulating under the leaves.
So - buy a few quiet osc fans - and vent thru a duct with an inline fan (if u can afford).
IMHO YMMV

Basically you want to move the air under the leaves