Cracked Exhaust manifold

I have a 97 Honda Civic LX and I noticed that there is a 3 inch crack in the exhaust manifold (looks like a hairline crack) - it appears to be affecting my gas mileage since the o2 sensor is mounted right on the exhaust manifold (30mpg rather than 40+ in my other civic) - but my main concern is the possibility of exhaust gasses leaking into the passenger cabin - how serious is the possibility/risk of that crack contaminating the air that the occupants breath?

I currently keep the vents closed via recirculation button staying on - and i often keep at least one window open to get new air in the car.

Lastly, it seems that replacing the manifold will be costly because the CAT appears to be bolted to the manifold and the replacement of both would cost as much as the car.

Thanks