Were there really any cost savings in Sinclair QL because of it being 8-bit design with 8-bit bus 68k CPU?

In complement to the other answers, never forget that in the 1980s memory was expensive.

For a 16-bit system, the base memory and every expansion costs twice the price of an 8-bit system - perhaps £100 more at the time.

This is a big reason why Intel sold so many 8088s (there were other cost savings in bus buffers and PCB).

Motorola tried to follow suit with the 68008, but just from looking at the instruction set, it was an unwise move. The 68000 instruction set is based on a 16-bit word, whereas the 8086 instruction set is based on the byte.

On a 68008, every instruction requires two fetch cycles, whereas the 8088 has a number of 1-byte instructions.

MOVE.B #0, R1 on the 68008 requires four fetch cycles, whereas the equivalent MOV AL,0 on the 8088 requires two fetch cycles.

So an 8-bit QL was cheaper on memory, and Sinclair probably got a good price from Motorola on the 68008 because it was too slow to be a candidate for a serious system.

Confession: I persuaded my father to buy a QL because I would never have persuaded him to buy the applications he needed, but also so I could learn about the 68000.

By the time the QL was first designed (starting as a "ZX83" in early 1983), a full-blown 8MHz 68000 was not a mass-produced, cheap commodity item, but rather a pretty expensive beast to buy. Common computers that had it at the time were the Sage and Sun range of workstations, the most common, "mass-market" computer that featured it was the Apple Lisa - Far beyond the QL's targetted market price. (There were others, like the Tandy TRS-80 Model 16, but basically everything that was a full package at that time had a 5-figure USD price tag attached.)

In 1983, an 8-MHz M68000 chip went at a mass-quantity price of roughly 40USD - Way too much for Sinclair's budget. The 68008 was about 1/3 of that. There's a famous quote from Steve Jobs, who went in negotiations with Motorola for the Mac, and was able to cut that going price for the CPU down to $9 apiece, but roughly half a year later when Motorola had optimized the production process, and, of course, with the Apple brand as a backing, rather than largely unknown (in the US) Sinclair.

When Motorola announced the 68008 at a much lower cost, apparently Sinclair saw the chance to earn a bit of the big 68000's fame with the much cheaper CPU and designed for that. Only during the development time of the QL, the 68000 saw a tremendous price reduction on single chip prices (A fact that opened up cost feasibility for the Atari ST, for example) - But then it was too late to re-design (I also pretty much doubt a 16-bit-design would have fitted into the QL's (physical and commercial) package: The Atari ST's PCB is about three times the size of the QL's).

TL;DR:

It was all about the incredible low price of 400 GBP. Outclassing any other 16 bit system (except for the TI 99/4) by at least a magnitude, on par or undercutting actual 8 bit machines as well. Every fraction of a penny saved was important.

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I've recently looked into the Sinclair QL [...] yet it is purely 8bit machine

I'd say it's an 8 bit system design, but a 16 bit computer.

With very little complication it could be rebuilt into 16bit machine: 68000 instead of 68008, maybe one or two more 74245 or 74373 chips, two or three 74xx glue logic chips and it even could still keep 8bit input into the video ULA (provided another 8 bits are buffered on 74*373). There should be, of course, some more pins for /UDS and /LDS byte strobes, ROM chip selects, etc., but that's basically all.

Doesn't that already sound like a lot? It adds up to several pound. More important, each of this chips does not only raise cost by itself, but as well by the holes to be drilled in every board, thus increasing PCB cost as well.

The PCB itself also does not look overcrowded, so it probably could be fit with 68000 and some extra more 74xx chips

Sure. But it would also have increased design cost. It's a huge difference between routing 8 data lines and doing so for 16. Not to mention additional signals. Professional design is not about cramping in as much as there is space, but as little as possible.

Provided those considerations and that it has ceramic-cased 68008,

Doesn't matter. I bet Sinclair did not require a ceramic package, so it's entirely plausible that it was Motorolas decision to use what they had - at a price point they agreed independently.

that was still advertised as 'advance information' in datasheets even in 1985

Many data sheets carried this label over 20+ years. Saying 'advanced information' or 'preliminary' in case of Intel was one way to avoid costly law suits with mighty customers (*1)

i.e. it was manufactured in little quantities, if any, and therefore was NOT cheap;

That conclusion is not supported by either. It needs concrete proof to make any speculation about price worthwhile. Price is, especially in startup production negotiated independent of production cost, keeping a focus on long term sells and time to market.

Were there any real cost savings for the QL as a whole from its 8-bitness, or it was just Sinclair's misconception?

Of course - like with all Sinclair products every penny counts.

The QL was set out at a very aggressive price point of 400 GBP.

• That's the same price as a bare bone BBC Model B
• It was half that the cheapest PC, a SANYO MBC 555 with as well 128 KiB did cost
• The new Mac was priced at 1500 GBP - almost 4 times the QL. And lets be serious, the Mac can't compete feature wise.
• A bare bone Apple IIe was 500 GBP, a design of 8 years before.
• Even a no name Apple II clone was the same 400 GBP
• Every Portable of the time cost about 400+ GBP(Tandy M100, Olivetti M10 Epson HX-20)

Not to mention that the, at the time most sold 68k desktop system, the Tandy 16/16B, cost about 6000 USD or ~4000 GBP (*3).

Sure, a Spectrum was only about 110 GBP and a C64 ~160 GBP but again, not really a competition here.

Bottom line: The QL was extreme competive priced and every hole, every chip, and every hour in design had to be cut. Going 8 bit external was a good choice.

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*1 - Stories that failed projects got ultimate blamed on supplier errors are a household issue. After all, nothing is better than telling management it was some changed or unclear spec from outside - and management tries of course to sue the supplier.

*2 - All prices are taken from a 1984 issue of practical computing.

*3 - Depending on the time of year. 1984 was a bad year for the pound, it lost more than half it's value against the dollar. Which in fact as well hit the QL.