I sell bricks at a brick yard
(Brickyard in Canada, although we sell bricks made in Canada AND made in the USA imported ones too)
Firebricks for inside the firebox of the fireplace are about
$ 1.40 to $ 1.60 each, and the surface that shows is 4 1/2" x 9", usually installed flat like a pancake, with fireclay / refractory cement, so you would need about 3.5 bricks per square foot.
Face bricks, for the face of the fireplace wall and structure can be anywhere between $0.80 to $ 1.20 each, most about $0.90 each. Coverage per square foot depends on which brick size, and that is usually anywhere from 4 bricks / sq.ft. to 7 bricks / sq.ft., and usually the smaller bricks look best, so count it as 7 bricks / sq.ft (smaller bricks need more bricks per square foot, but looks best especially since its a relatively small wall you see close up, instead of a whole building you see from a distance).
These are actual true prices (retail and contractor prices) for the brick themselves, before taxes, and delivery costs. These prices are for just the bricks, not including installation.