IMO most nail polish I have bought is cheaper than Testor's enamel, and much greater color assortment, BUT not necessarily the colors that you are looking for: lots of pinks & reds and pearly/shimmery/sparkly "other colors".
Also found that they can often be more transparent once on the frame than the opaque enamels in the Testor's jars, but since they are basically lacquer they dry faster and can be removed with acetone or nail polish remover, usually causing no harm to the parent paint.