Even though designers dominate the fashion industry, people should never forget that fashion is not all about designers. It is basically clothing, trends and how you style it! Ralph Lauren said it best "Fashion is not necessarily about labels. It’s not about brands. It’s about something else that comes from within you."
You can wear designer from head to toe and still look hideous, whereas someone wearing non designer clothes can look like a million bucks!
And please don't get me wrong, I have nothing against designers. Actually I have a couple of designer goods on my wish list myself but I mean there is nothing wrong with mixing things up.
If you are on a budget, but still wish to invest in some designer goods, to me the designer goods that are totally worth the investment are mainly accessories, like sunglasses, shoes, handbags and watches (jewelry). Since handbags tend to be more expensive my advice will be to find classic or limited edition pieces of that particular designer. In this case your bag will be "classic" & not "last season" as time passes.