Covering your plate is *definitely* not an etiquette rule, and isn't even practical. Unless the happy couple invoices you on the invitation, you have no idea how much the reception costs. There are huge variations in catering costs depending on where you live, what you are serving, and the alcohol options. You could, I suppose, go to the wedding first, and send a gift later based on the quality of the hospitality and how much alcohol you consumed....
I would say that in general, if you attend a wedding, you need to give a gift. But you *don't* need to buy off the registry, and you *don't* need to hit a particular price point, particularly if the budget is tight. If money is tight, go for something inexpensive but carefully thought out, and if the bride and groom object, they are showing their own rudeness.
If your friend is paying for the other attendants' attire, but not for yours, then I think she is being rude, unless she's telling you just to wear your normal dressy clothes. I think it would be fine to counter with "Listen - my budget is really tight, and buying a new outfit for the wedding is difficult. Can I just wear (clothing I already have)?"
Potluck weddings, as others have said, are rude to begin with. As you're in the wedding party, I'd say bring something pre-packaged so you don't need to fuss. A couple of bags of tortillas and some jars of salsa would be fine.