Homeowner, the price of a new vent. Plumber, $100 to 150. Just shut down the boiler water feed, release the pressure and drain water below the valve, unscrew the air valve and put a new one in. (I assume it is not soldered into place) Because it is in the attic you would not have to drain more that a quart or two of water. Then when you turn the supply back on bleed the valve.