Commercial press printing
Most of us are intuitively aware of the dependency that the per copy price falls with the increase of the press run, but are too lazy to go backward and reason that the per copy price rises (in fact, even more steeply) with the decrease of the print run.
A simplified explanation follows: the average print house employs big machines run by operators, and it takes technological time and efforts to make the machine ready for printing. This preparation usually is cited as a fixed "set-up" cost, which is much bigger that the per copy cost of the printer.
The economy of scale here works as follows: the average per copy cost is calculated by adding the printer per copy cost to the inital set-up cost and dividing this sum by the print run. Thus the longer the run, the lower the per copy price. Conversely, to print just one copy you add the this time negligable percopy cost to (again) the entire initial cost and divide by 1. The result is a HUGE value. Of course, this example is exaggerated, since no one is even thinking of printing one copy of anything with such enormous overhead. However it illustrated the point that there exist print runs - short print runs - for which the use of a commercial printer is infeasible.
The alternative – digital printing
An alternative to the above process with short print runs is the so called digital printing (for the time being think of it as a laser printer or copier). It is characterised by much smaller inital or preparation cost and comparatively higher per-copy costs, which may be thought of as a constant.
Take the shortest of print runs and the simplest home and/or office printers. Their per copy costs at 1 or two copies are understandably more than reasonable, otherwise we would not be using the home or office laser or inkjet printers. Add colour and/or paper size (e.g. A3 or more) and or numerous copies and thousands of printed pages and the SOHO printers will not manage.
Another simple example of a digital printing environment is provided by the copy shops, which offer relatively low per copy price comparable to that of the SOHO printer. Especially for just a few copies of a sheet or two. However, if you take e.g. the average copy centre per copy price and extrapolate to an entire book, the per book price becomes prohibitably high. Of course, it does not matter for a copy or two or even for a few hundred, since there is no other choice, but obviously print runs of thousands will become devastating economically. This is the realm of the commercial presses, discussed above.
Let us summarise our observations so far: for small print runs digital printing is preferable to press printing economically, and at a few thousand copies or higher the commercial presses easily beat the digital printers. Think of the per copy price of a given printer or a press as a curve. Le us superimpose these two curves on a common sheet. In this case we have two curves which move in the opposite directions and surely cross somewhere. But where?
Where is the "press or digital" break-even point?
So far we have come to the conclusion that the general job division between digital and press printing is as follows: presses are for log-run printing and digital is for short-run printing. What is not clear is the area inbetween: 50, 100, 300, 500 copies. Which one is cheaper?
We can look for an answer by superimposing the two curves, discussed above. The break-even point, where the diminishing per copy price of the presses crosses the line of the relatively constant digital printing per copy price. At this point and the correponding print run the per copy prices of the two technologies are equal. Unfortunately, this dies not provide us with a universal value to compare the two technologies. Different companies (even on par technologically) often employ different pricing schemes and an exact value can be arrived at only by comparing two specific offers. With a given pair of companies/technologies the break-even point may be at a print run of 100, with another - at 300, etc. Therefore each separate case must be treated on ots own.
The response time factor
An additional benefit of digital printing, which cannot be quantified into the same diagram is the set-up time (technology-wise), or time to finish the job (from the user point of view).
It is not uncommon for the average press to take a month to produce a book (of course, depending on work load), while in similar circumstances the digital printer can finish the job within a week or a few days.
This issue understandibly takes part in the decision press or digital, since if you do not have the time for the press process you have no choice but to print digital, even with longer print runs.