You are 100% correct.I don't think furniture is cheaper in the "capitol" SD. For DR-made items Santiago and surrounding area, going directly to the manufacturers is where the best deals are IMO. Depends really on what you want, if you buy imported furniture it matters less.
Santiago is as good, if not better for prices.
Santo Domingo may have more shops and more selection, but not better prices.
I think the rents might be too high there, and it drives the prices up.
Either way around, there shouldn't be much of a difference between the 2, certainly not 25%.