Our products are typically priced per piece and the unit of measurement can typically be found next to the price.
Having said that, this tread is Caribbean Heart Pine (which is a much more expensive than some of our other species) and is prefinished with our Aluminum Oxide top coat. Our most popular products are made from Red Oak. Here's a link for the same treads in oak:
http://www.stair-treads.com/retro-treads-and-risers/replacement-stair-treads/closed-box-treads.html?length=18&wood_species=68
We typically offer some of the best pricing in the market on stair treads because we manufacture all of our own products, but some wood species by nature are going to be more expensive than others. If you're looking to keep the cost as low as possible, consider an unfinished product as well - they are priced lower but you'll have to apply your own coat of finish (polyurethane or equivalent) once you receive them.