Let me take a stab at this -
I work in sales - I used to sell phones, then cars, and now I sell houses. If I'm meeting just an average joe or working in the office I wear a polo shirt (white, blue) and shorts, typically khaki color or something else neutral and tennis/casual dress shoes - basically "golf" clothes. This is in AZ, so lots of people there dress this way, therefore it's accepted. If I'm showing investors around or if I have an appointment to show an expensive house then I wear a suit, tie, and dress shoes - now, most of the investor types and high-dollar people I meet don't dress this way but they do expect it.
When it comes to dating you are in the same boat - if you're just s college kid and everyone is poor then target clothes/salvation army whatever is fine, but if you are older and/or in a more professional or wealthy area then you dress accordingly.
Will wearing expensive clothes make you attractive? This is a difficult one because the answer is both yes and no - on one hand it will, but on the other hand you will still be you. So while you may look like a guy out of a magazine, or wearing the same threads _____ actor had on in ____ movie, you aren't that guy in that magazine or that guy in that movie - you're the same you that had on a tore up pair of jeans and an old Bob Marley shirt yesterday, you just look more respectable. And if you are breaking the bank and running up credit cards to buy clothes just to impress girls that's irresponsible at best. It's really no different than buying an expensive car or something like that in order to impress people - a friend of mine signed his life away at age 22 to have a BMW and tailored suits to impress girls. Now he's 27 (single of course), bankrupt, with -900000000 credit rating - do you think it was a wise decision?
Slob? Wearing cheap clothes does not make one look like a slob or unattractive per se; poor hygiene, dirty clothes, stuff that doesn't fit right, all that make you look like a slob.
If you saw a pretty girl wearing a t-shirt and tight jeans, would you dismiss her because those jeans didn't look like designer ones to you? Conversely, if a girl saw you (and thought you were attractive) but dismissed you because you weren't wearing a tailored suit, would you really want to be with her? Maybe I'm just old fashioned but I think what's inside those clothes counts for more than how much $$$$$ it took to get them.
So in the end 1) dress for your audience 2) if you can't afford those expensive clothes just wear decent, clean stuff that fits and 3) take care of your hygiene. That's really the best you can do.