Bump ....

My recommendation would be a second zone for the basement with a return. It doesn't take much to heat a basement so the equipment does not have to increased in size.

The other option to save money is to use space heaters for those few times you need heat. You will be surprised how few BTUs it takes to raise the temperature a few degrees.

I don't like the idea of electric heat either baseboard or portable space heaters for nice new construction. You could consider a mini split or zoning control to heat the basement area independent of the main floor zone.

IMO

I would do electric to supplement the 1 zone HVAC if necessary but my wife is adamantly opposed - like tigerdunes.

Anybody have any insight into whether $4500 is a reasonable cost to add a second zone. It's about 3x more than I expected. The entire basement is only 2100 sqft and we are finishing 800 of that. The amount of extra duct with the second trunk would be minimal so I am thinking most of the cost has to be in the controls for the second zone.

It's not reasonable. It's very typical for builders to have high mark ups for an upgrade. Upgrades generate good profits.

You could add the duct work with a manual damper. You would open the damper on the days additional heat is needed.