Before going more indepth with your question, I would like to clarify it. Because the first half of your question make it sound like you are asking how a printer financed the construction and setting up of their printing press and print shop, but the second half is obviously referring to the output of the presses. These are two separate concepts which weren't necessarily connected financially.
So to answer both iterations of your question, although the first one only briefly, as I suspect the second form was your main one, the first one being how printing presses were financed, the answer is the very simple: If the printer did not have the means himself to set up a printing shop he would find investors who would loan him the money, most often in exchange for annual interest.
Infamously Gutenberg lost his first printing shop to the investor Johann Fust, when a dispute over debts ended in a court ruling in favour of the latter. Most printers by far in early modern times weren't wealthy people, and the only way they could secure an income for their old age would be to sell their shop, because they wouldn't have made enough money to have much in the way of pension funds through their work. That is why, if the printer died in his prime, you will often encounter his widow taking over the business, not infrequently either marrying or just hiring a masterprinter to take over affairs, because the husband simply didn't leave anything else for her and her children to live of off.
The second part of your question regarding how a product of the printing press was financed is more varied.
Your suggestion of "pre-orders" isn't entirely off, as subscription printing was a thing in early modern times, however it is a relatively late invention, not appearing until the 17th century in England (Raven 2007, 105ff), where it quickly spread to the rest of Europe.
It functioned through the publisher collecting subscriptions to an upcoming work, and when enough subscriptions had been collected the printing could commence. Most books were very local affairs, with subscriptions only being collected in the local area of the publisher (a city and its surrounding areas, or the entire country), but some were very impressive international projects with subscription collectors being hired (for a percentage of the subscription fees) in numerous countries. Robert Darnton has described the enormous publishing projects of the various editions of the French Encyclopedié in the 1770s-80s, which involved subscription collecting in basically every European country from Spain to Russia (Darnton 1979).
However, as I mentioned, this method of financing books was a relatively late invention. It can be argued that its invention was an important vehicle for the massive increase in published books which occurred in the 18th century, because it made publishing much easier, as in theory everybody could publish a book regardless of how wealthy they were (still by far most titles proposed for subscription never saw the light of day).
Book printing was expensive, until the invention of woodpulp paper in the 19th century, all paper used for book printing was handmade linen paper, which was usually by far the most expensive part of book production (Darnton 1979, 185), even compared to the cost of labour, which was relatively cheap at the time. So anyone wanting to publish a book needed to have funds ready to pay the papermakers, long before any printing had started. The printer and his staff of course needed to get paid as well, so all in all it required a decent sum of money to print a book. Before the invention of subscription most authors who were not wealthy (and that would have been most of them) therefore needed wealthy patrons to finance the publication. This is why you so often see huge dedication pages in books from the 18th century and earlier, because the people whom the book was dedicated to either had paid for its publication beforehand or it was written in the hope that they would present the author with a monetary gift as thanks and thus help cover any expenses incurred in its publication.
In this the book printer simply functioned as an artisan, taking money to print a certain amount of books for a set sum which covered all expenses including salary, with all the books (except for a couple of copies which were traditionally part of the payment to the printer and his staff), and that would be the end of any involvement by the printer in that transaction.
However printers of course also functioned as publishers themselves, and the printing of bibles, hymn books, school books, prayer books, chap books and all sorts of potentially popular titles constituted an important supplement to the simple payments for printing jobs as outlined above. And in those cases the printer had to cover all expenses himself. So any books where the printer was the publisher would be considered a sort of investment of the printer into his own business, and it was a gamble whether that investment showed any returns. That is why they most often chose titles which were almost always sure sellers without being very capital demanding (small popular devotionals and chap books as opposed to multi volume scholarly works), like the abovementioned. Again, there are exceptions, and printers like the scholar Aldus Manutius or the Elzevier dynasty managed to become relatively succesful publishers of much more expensive titles, but this either required a starting point of wealth, or it had to occur gradually as they became more wealthy and thus had more money to invest in more expensive titles. And by far most book printers never saw anything near such wealth in their lifetimes.
So in summary, before the mid-17th century, all printed books had to be paid up front by somebody, making even printed books a relatively expensive luxury item for the most part. From the 17th century onwards subscriptions, serialisation, selling shares in upcoming books as stocks and lots and lots of other funding techniques was invented (most of them in England, from where they spread to the Continent), making publishing not only more widespread but more accessible to people who were not necessarily wealthy.
However it was not until the industrialisation in the 19th century, where the invention of wood pulp paper, mechanised presses, type foundry improvements and so on, could introduce a truly cheap mass press form of publication.