What the price is for
A slant-back box vent, also called a louver, turtle or static vent, is an aluminum hood over a square hole in the deck, with a flat flange around it. The estimate prices 10 black aluminum units, a bundle of shingles, roof sealant and nails, plus 8 person-hours at $78 an hour.
The bundle of shingles, $30 to $60, is there because fitting each vent means lifting the courses above the hole so the top half of the flange can slide underneath. On an older roof the tabs have bonded down and some crack when lifted, especially in cold weather, so the crew swaps them out.
The figure leaves out intake at the eaves, removing old vents and patching their holes, wiring for a powered fan, rotten deck found at a cut, and the extra time on a steep or two-story roof. One vent on its own usually costs more than the per-vent figure, because many roofers have a minimum charge for a trip up the ladder.
Box, turbine, powered or ridge
Box vents have no moving parts and nothing to wear out, and each is a small hole, so they suit a roof that needs exhaust added without disturbing the ridge. Each one draws only from the area around it, so a roof needs a row of them. They are the usual choice on hip roofs, where the ridge is too short for a continuous vent.
A turbine spins in the wind and pulls more air than a box vent while it turns, but does little on a still day, and its bearings wear. It installs much the same way, a round hole and a flashed base, so it costs more mainly because the unit does. A powered attic fan moves the most air and carries the most risk: unless intake is generous, it makes up the shortfall by pulling cooled air out of the house through ceiling gaps. A wired fan also needs an electrician, which this estimate does not include.
Where the roof has a long, unbroken ridge, a ridge vent is usually the better exhaust. It runs the whole peak, is nearly invisible and has no base to flash; see ridge vent installation cost. Do not add box vents to an attic that already has one, because the two short-circuit. Types of roof vents compares every option.
How many box vents an attic needs
Count from net free area, the open area left after the louvers and screen. IRC R806.2 asks for 1 sq ft per 150 sq ft of attic floor, reducible to 1 per 300 under certain conditions, split roughly evenly between exhaust and intake. Box vents are typically rated somewhere around 50 sq in each, and the exact figure is printed on the carton.
A 1,500 sq ft attic at 1 per 150 needs 1,440 sq in in total and 720 sq in of exhaust, which is about 15 box vents. An attic that qualifies for 1 per 300 needs half that. The roof ventilation calculator runs the numbers for yours.
Past the point of balance, more vents make things worse. Exhaust that outruns intake pulls air from the house, so check the soffits before adding anything up top. If they are solid, soffit vents are the first spend.
Why the holes go where they do
High on the slope. Hot air gathers at the peak, so box vents go in a row a short way below the ridge: close enough to reach the hottest air, far enough down that the top flange tucks under a full course of shingles. A vent halfway down the roof exhausts cooler air and leaves the hottest pocket where it was.
Between the rafters. Before cutting, the crew checks from the attic or reads the framing from the nail lines so each hole lands in an open bay, clear of rafters and truss chords, wiring, ducts and bathroom fan lines. A hole that would hit a rafter gets moved; the rafter does not get cut.
Spread along the roof, at one height. Vents spaced along the full length give every part of the attic an outlet, where a cluster over one room leaves the far end stagnant. Keeping them in one line stops a lower vent feeding a higher one. They also stay clear of valleys, where runoff concentrates, and when both slopes serve the same attic they usually go on the one that faces away from the street.
Ask to see the planned positions and the count arithmetic before work starts. A count worked out from your attic floor area and the vent's rated net free area, with the soffits checked, is one you can trust.