I do not think that it would be an easy calculation to make and way to many unknown variables.

The cost difference
the houses energy performance and requirements
how well you manage the system

That is a question that needs to be answered for your situation.

However we have a 2 zone system here and it saves a lot of money as the lower level we haven't needed to heat or cool all year. So we fundamentally only heat the main level.

Friends of ours did a 2nd zone in their master bedroom so they keep that more comfortable and essentially live there in the winter.

Other people will do a zone for the bedroom and a zone for the main living area and set it up with day/night thermostats so they heat the areas where they live when they live there.

Others like cooler bedrooms as you sleep better so they keep the main living areas warmer.

I don't know how a 2 zone system would cause more maintenance issues than a single zone...it's not like it puts a big strain on the system. It only means you have 2 thermostats which are pretty reliable.

I think its the dampers that eventually cause the problems. Locate them in an easily accessible area which is usually near the air handler. Zoning is certainly a tricky issue depending on the details.

Not sure about your mild climate but its the two-stage compressor and variable speed air handler that saves the most money by better controlling humidity.

Generally a 2 zone system doesn't save enough money to justify the expense.

CA can be different however.

If you are using NG to heat and you are talking new construction, zoning will definitely not pay off. If you heat with electricity in an old house, it probably will.

I zone and haven't had any issues. I do it to sleep at 65 or lower while not freezing the rest of the house. It is a comfort issue. It probably saves some money but it is a large house and I wouldn't have done it for that reason.

We've always had 2 zones, 1 for each floor with the bedrooms being upstairs. We had a programmable thermostat and programmed it to only heat/cool the floor we use during the day/night.
We also looked into upgrading our hvac to 15 seer with a variable speed air handler to make it more efficient and that would have added $5000 to the cost.
But they came back with the option to leave the hvac 13 seer and do a variable speed air-handler with a humidifier and that would have added $2800. Another option was the same as above but instead of variable speed just do multi-speed air handler with the humidifier and that would add $1800 and it would be the equivalent of 14.5 seer. So that's what we are going with to get the most bang for our buck.

I thought it was minimum of 3 zones up to 5 zones.

anyway, as to savings...
as you mention not heating/cooling some areas
like living are at night has a savings.
comfort is a great thing with zoned systems
because you can control temps by tstats
rather than one tstat controlling whole house.

eventually all motors fail. motors on zone
dampers are not excluded. any early
failures would be covered under warranty
& you may have to pay labor to change.

you'd have to weigh cost of zone equipment
control board, motorized dampers & multiple
tstats. against cost of install without zoning.
factor in things like vs ahu's, two stage equipment
etc to get the numbers to compare.

another issue is getting a contractor with experience
in zoned systems. I've always found a zoned system
to be great & homeowners love it...but if the contractor
hasn't done a zoned system, or uses a different brand
than they are used to..it can be a disaster.

in any event, compare the numbers
shop for experienced contractors &
get opinions/bids from them.

it will be easy to tell who understands
zoned systems & who doesn't once you
talk to a few companies.

best of luck.

Rather than heating the bedrooms much, try small heaters in the bathrooms and electric blankets.

It's silly to have an entire floor warmed so you can sleep with no blankets.

We have three zones and it works well. I don't really know if it saves money or not.

lazygardens - It can be a comfort thing, not everyone can get a good nights sleep under an electric blanket.