It could be dramatically more expensive to make an M1 chip and Apple is making the trade off for performance/walled garden security.
Unlikely, but possible
But Apple didn’t switch chips for the cost. Just like the PPC to Intel switch, they did it for the performance and efficiency. More efficient means smaller, thinner, less battery, etc... more design flexibility. That and the fact that they fully control their release cycle is also a nice bonus.
Please also note that a modern non-M1 Mac included a TSMC silicon (T2) on a similar order of complexity (A10-class) anyway that’s no longer present (albeit at a larger node).
Me personally? I have no way of knowing or estimating those costs either way. But that was how I interpreted the parent comment.
If you allow that R&D costs for the M1 are largely aligned with the A* series for iPad that they would have already been paying (from your comment), then you're left with the manufacturing costs.
For Intel, they have: 1) chip design R&D, 2) fab method R&D, 3) fab costs, 4) sales/marketing/profit margin
By switching to their own chips, Apple has absorbed #1 and #4, and left #2 and #3 to be outsourced to TSMC. If the main costs for #1 are duplicated from the iPad chip, then you're left with fab costs (and fab R&D).
But, again, even with the likely cost benefits of making your own chips (and avoiding the Intel profit margin), I doubt that was the deciding factor in switching. The costs probably made the switch possible, but wouldn't have been the primary factor. Apple would gladly pay more per chip if it was more efficient (heat and power) and if they could control the release schedule. Even if their total costs for a single M1 chip are more than for an Intel chip, they probably still would have made the switch.
Apple has only taken over the design of the chip not the manufacturing, Intel was doing both for Apple. Nobody has any data to know how the manufacturing costs have changed.
There is real chance that total costs for Apple could have increased overall and there is no way to know. Intel was in difficult position over the Apple deal for years, Apple would have negotiated down significantly given their threat to move.
For all we know Intel was selling at loss per chip to apple and making it up in other business.
The vertical integration of all is frightening though. It does not end well with many industries. You need competition and being now on arm and one gpu meant all those fierce competition in pc and gpu of intel / amd / nvidia is just figures. You do not get the insight and innovation by looking at how good or bad other side is.
There are some limitations: lack of PCI-E lanes, lack of off-package RAM support, lack of user-replaceable RAM support, lack of discrete graphics support, lack of Thunderbolt 4 support. These are all essentially non-performance features of the chips that need to be developed to bring them up to feature parity with the Intel chips.
This generation happens to have carefully bypassed the need for those features, a smart move by Apple for the first release, but later releases for the higher end MacBook Pros, the iMac/iMac Pro, and the Mac Pro, will need most/all of these features, and those will likely take significant time to work into their architecture.