if 1 oz. = $20 how much is 1 ml?

1 oz  =  29.5735 ml   (approximately)

So:  1 oz / $20.00  =  29.5735 ml / $20.00

You can set up a proportion:

--->    29.5735 ml / $20.00  =  1 ml / x

cross multiply:  29.5735 · x  =  20.00 · 1

--->                                  x  =  $0.68