Negative Pressure necessary to remove odor from grow?

If I have a grow in a closet and it is fairly important to have zero odor, if I just keep exhausting more than i draw, should there hypothetically be no smell? How much does it cost to keep 2 fans running 24/7 cost?

Where are you exhausting to? Is it just back into the closet, or through the closet door into the room? If you are exhausting to the "outdoors" you should be OK.

With carbon filter I hope

The electricity costs associated with vent fans is negligible. I have an 8" fan set to a low setting and it only uses 40 or 50 watts, which is just a few bucks per month. The lights and dehumidifier are my big electric costs.

If you're fans is 100w it'll take 10 hours of use to consume 1kw.
24 hrs x fan wattage dividided by 1000 x unit price = cost.

Fans don't use a lot of electricity my 6" rvk is only 100w, I've got it turned down to around 30% for noise issues.