I believe this question would be complicated to answer even if we had traveled back from the future where ARM-based Macs were already available.
There's a lot of assumptions that the product lines, form factors, etc will stay the same and that Apple will just sub out the Intel chipset for Apple/ARM.
What is more likely to happen, imo, is that the entire product range will change over the next ~3 years. Better differentiation between form factors (thin vs powerful/longer battery) - with different price points to match. I imagine the ASP of Macs won't change that much, with high-end price points still existing to cancel out the introduction of lower $799 entry level price points.
Gross margins for Mac will be higher, but entry price points will also be lower. Apple may also use some of the Intel cost savings to introduce higher cost materials, displays or designs. When the Aluminum unibody was introduced, it cost apple hundreds more. This would be a fantastic opportunity for Apple to come out with a new, more expensive, design that would be offset by lower chipset pricing.
A decade ago, a MacBook that was priced around $1100 or so to the consumer would cost Apple around $700 to make. An iPhone that sold for $650 would start off at a cost of around $350 at launch and then decrease over the course of 6 months to $250. Costs at launch are always higher.
There are so many possibilities here - hoping, and optimistic, for a creative Apple line-up with some new innovations.