Mathematics has always been a bit beyond me, but I take a simple pleasure in arithmetic. I remember, in the course of a long solo cycle from Fort William to Mallaig in the pouring rain, amusing myself by ringing my bell at intervals and betweentimes calculating how far a single turn of the pedals in my top gear would take me, and from that how fast I could go if I pedalled at 60 rpm.*
life is an uphill struggle at times
The printer’s quote of £225 for the makings of 20 books gave me a figure to work with. Suppose I could make 100 books for that price? That would bring the unit cost for printing down to £2.25 a copy, leaving £12.75 change out of a selling price of £15. Surely I could supply all the bindings for that and still have some over? I might even make a reasonable profit…
The question of profit per book is one that exercises the minds of most writers, largely because it is so small. The royalties on my first book were 7.5% for the first ten thousand copies rising to 10% thereafter, which amounted to about 67p a copy, rising to 89p. It does not take much calculation to realise that you would need to sell a lot of copies to earn a living at that rate.
gratuitous picture of my first book in a variety of tongues (I like the Italian cover best)
What if, I asked myself, I could make a book, all in, for £10? Then I would have £5 clear profit per book – such unimagined wealth that I could afford to cut a deal with my friend Shaun Bythell for the use of his premises and access to his sales network and still have something to spend on drink.
So, the new target became £10: could I make a book for that, if I aimed at a total production of 100 copies and (optimistically) sold them all?
I knew what £225 would buy me from the local printer – twenty books, unsewn and unbound. It occurred to me that quite possibly I could buy a monochrome laser printer for that – one that did duplex printing – and so print as many copies as I wanted.
It did not take me long to discover from the resources of the internet that I could have one for £210 which met all my requirements, being economical in operation and capable of printing double-sided.
£2.10 per book would buy me a printer; that left £7.90 for all the rest.
All I had to do now was work out what ‘the rest’ actually was…
*The basic calculation is no. of teeth on front chainwheel (say 48) over number on rear sprocket (say 16) times wheel diameter (28″ in my case) times π (which I usually took as 3 for ease of multiplication) – 3×28=84″ or 7′ times 3 = 7 yards per revolution, so 420 yds a minute, a bit under a quarter of a mile (but remember that you’ve rounded π down) so about 15 mph – by which time you are a good bit further on your journey. This calculation was rendered all the more diverting by my peculiar gear set-up, which involved a Sturmey-Archer five-speed hub fitted with a double sprocket, allied to a double chainwheel – an arrangement which necessitated no fewer than four separate levers to operate, and a great deal more arithmetic to calculate – for instance, the top gear on a Sturmey-Archer 5 speed is 3/2 or 150% of the middle or direct gear, so with the set up already quoted, 84″ would be geared up to 126″ or 10′ 6″, giving ten and a half yards per turn or 630 yds/minute, about 21.6 mph.
COMPLEAT TROWZER 
I didn’t know that one of your books had been published in several languages. Well done you! And PS… Math makes my head spin.
Yes, one of the things my first publisher was good at was selling foreign rights… it was fisrt published simultaneously in French & English, because he was based in Montreal (though maintained an office in NY state so that he could claim to be ‘a New York publisher’) and had received some support from the Arts council or equivalent. It was subsequently published in German, Spanish, Italian, Greek, Turkish, Polish, and Serbian (in which my name was rendered as Dzon Vord)