Sexy: modest décollété
Cheap: Australian cleavage
Cheap, Vulgar: microbikini
Sexy: short skirt
Cheap: skirt so short that you can't sit
Sexy: Daisy Dukes
Cheap: Cut-offs with hiney partially showing
Sexy: wearing a unexposed thong
Cheap: showing a whale tail
Cheap, Tacky: t-shirt that reads "Voulez-vous coucher avec moi ce soir?"
Whether wearing a bra or not falls into one category or another is usually seen depending on whether there is a hint of the nonwearer's nipples or not. (Sorry for my crass candor here.)
A few years ago a Minneapolis company marketed a product called Bodyperks: these are nipple enhancers that could be placed under one's bra or worn directly on the breasts while not wearing a brassiere. In effect, nipple falsies! The web site below introduces the product, contains a number of testimonials from happy users, and illustrates how it might be used.
In the television program, Sex and the City, this was mentioned in one of the episodes as the Manhattanites strived to push the limits of sexiness as that bit of televised froth dramatically portrays.
Appearing on any scene while wearing these Bodyperks would, to say the least, draw a considerable amount of attention! This is because the product gives the viewer the impression that the wearer is overperky and also somewhat sexually aroused as well. (Nipple erection can accompany sexual arousal or just plain being cold.) These Bodyperks are a specific instance of what Niko Tinbergen called supernormal stimuli: exaggerated versions of stimuli that normally produces a response, but becomes preferred to the normally-occuring stimulus.
In a real sense, if in the competition to seek mates (or at least to have one-night stands) more women employ such prosthetic means, then there will be pressure on the others to do likewise! This might, therefore, result in a breast-enhancement arms race!
In my opinion, this is way, way over the top. Far beyond tacky!