I agree with people saying that the prices won't change.
That being said, if Apple decides that there is redundancy in their lineup (e.g. with 13.3" laptops and 27" desktops), it's possible that they may replace the lower-end of the two similar Intel machines with a single lower-end ARM Mac that has specs that rival or surpass even the higher-end of the Intel ones.
This may not be likely with the iMac Pro and the 27" iMac considering that these, under the hood, are two different classes of computers. But, for all we know, maybe the next Mac Pro brings the cost down to iMac Pro levels and cannibalizes the need for the iMac Pro. On the 13.3" laptop side of things: Considering the main difference between a MacBook Air and a 13.3" MacBook Pro, aside from a TouchBar and possibly two extra USB-C connectors, is a processor that needs a larger thermal envelope on the Pro and a weaker smaller one that can make do with less of one, it's completely possible that Apple will just merge the two laptops. If we got a MacBook Air form factor, with speeds on the Apple Silicon CPU that put every 13" MacBook Pro to shame, and all other 13" MacBook Pro features in tow, would we really get more out of having a separate model?
So, yeah, if they make a MacBook Air on ARM that handily eclipses both the Intel 13" MacBook Pro and the Intel MacBook Air and has the same ports and TouchBar, I could totally see that being sold at MacBook Air pricing (which is cheaper than 13" MacBook Pro pricing). Similarly, if an ARM Mac Pro brings the Mac Pro to iMac Pro pricing (because the Apple SoC is cheaper than the Behemoth Xeon processors), but is expandable, what need do we have for another iMac Pro? Or alternatively, if the next 27" iMac replacement bring the iMac to iMac Pro territory in terms of performance, why do we need two iMacs?
Apple has the ability to change the lineup throughout this transition much in the way that they did when moving to Intel (which saw the merger of the 12" and 14" iBook into the 13" Plastic MacBook, and the removal of the 12" PowerBook). They can totally do it again. But, barring things being incidentally more inexpensive, no, Apple's costs on things won't be cheaper.