Buying wood from a sawmill can be exponentially cheaper than buying from a big box store. But sawmills work a little differently, and if you're not familiar with how to purchase; it might trip you up.
First, let's talk about the obvious differences. The big box stores take pieces of wood that were grown in mass to be cut-down and used as project lumber. Usually this is limited in species to oaks, hickories, maples, walnut, and maybe a few others. These boards are rarely straight, but are sold in identical lengths. So, a 4'x6"x1" board of oak will all cost the same price because they are all cut to exactly the same width, height, and depth. This makes it so that you can go to the register, scan a barcode, and checkout.
At a sawmill, there are no barcodes. This is because there is rarely and consistency between boards. Sawmills will simply maximize the amount of wood that can be cut from a log in a particular sawing pattern.
WOAH. Wait... 'Sawing pattern'?
Here is where sawmills shine. Not only will mills have vastly more species than the big store, but they will likely have live-edge pieces, quarter-sawn, rift-sawn, and plain-sawn (what most do) of many species. Having this immense variety means you really need to know your stuff and do your research. You need to know what species you want, what species they offer, and what kind of cut you would like. Some saw mills will even carry exotics like zebra wood and Peruvian Walnut that do not naturally grow in your area. Talk about information overload. Always call-around to your local mills before you make the drive. Depending on the size of the mill; they may have their own land that they cut from, which means that they can be far-removed from urban society.
Once you know all of that, you need to understand the concept of a BOARD FOOT.
A board foot is usually one inch thick, less than 6 inches wide, and one foot long. Mills will charge you by the board foot, and will have additional fees for pieces thicker than one inch or wider than 6 inches. So, let's say that you get to your local mill and want to buy a board that is 4 inches wide, an inch deep, and 8 feet long. That is 8 board feet. That particular species is sold for $4.50 a board foot. That means that your board will cost you $36. Now, that might seem like a lot, until you think about it being a much nicer wood, a cleaner cut, a straight board (good luck finding that at a big store), and there are cracks on either end that the mill wont charge your for. So, now you're only paying $24.
This is a huge savings compared to a store that might have a barcode of $60+ dollars for a similar piece, but it requires a little more brain-work, and might require some hunting to find all of the sawmills in your area. Don't let it scare you though. Mills are usually local operations and thrive from your business. Most will even help you find the right species and size for your project. Most are a quick google search away.
As Always; Make Something Awesome!