"To" is used with a specific value, as you have done. You would not say "he came down on 12,000 dollars." You would not say "he came down to the price."
You could say "he came down to the price I wanted." Again, that is a specific thing, "the price I wanted."
OK..so "to" with the price, with the value or in the expression "to come down to the price", and what about "on"? Can "on" be used only in the expression "to come down on the price" or do you use it even in other particular expressions (always with "come down")....?