Timber lengths for DIY

Also depends what you are buying and its intended use.

If you want everything planed on 4 sides to exact dimensions then the processing is expensive and you are paying for a larger chunk of timber than you actually get as someone has to pay for all the wastage from the cutting and planing process.

Timber quality also has a massive impact on price. Timber with few or knots or no knots larger than a certain percentage of the width of the timber will also be more expensive. Longer lengths can be more expensive. How the timber is cut makes a difference as does how well it was grown. Look at the end grain some lengths will have very close growth rings other lengths will have very few.

Some of the cheapest timber is often rough sawn treated timber used for outdoor projects making garden sheds and the like. The reason possibly being that cheaper timber pumped it full of preservatives is just as good outside as quality close grained timber which won't absorb as much preservative. Any rough sawn timber will normally be cheaper than planned all around timber but you can rarely guarantee dimensions. btw don't expect 16 ft lengths to be 16 ft :-) Its normally 3 - 4 inches sort normally its 480cm or 189 inches.

Packs of timber are also often more expensive as they are packed for the retail market.

Bottom line is unplanned timber treated for outdoor use is often the cheapest.