Don't panic. It's not a trick question.
There's an insidious seduction to a pay-what-you-can night at a show that's a little out of your price range. On the one hand, you get to see a show you would otherwise have to miss, but on the other you don't want to seem cheap, or like you're not just as appreciative as the big spenders. Fear not! Theatres with PWYC have not booby trapped their ticket prices, and offer PWYC nights or ticket tiers specifically to make shows available to everyone and attract a wider audience (e.g. broke ass art punks). There are a few things to understand about how ticket pricing works, though, so if you're still dubious, read on.
Most events will have a minimum ticket price listed (something small, like $5), as well as a suggested price, which is usually still cheaper than a full price ticket. If you can pay the suggested price, please do! Supporting local artists is one of the best things you can do to directly improve the quality of living in any community. If the suggested price is going to keep you from attending, it's fine to pay less, and it's also fine to pay the minimum if that's your budget.
Here's the important part: The whole point of PWYC seats is to get new people and new kinds of people to risk a few bucks and a Thursday night on some art. If you enjoy it, recommend it! Tell your friends to go, or see it a second time and take them. Tell your mom about it, post it to your Instagram, brag about how good it was at work in front of that one girl who's fiance took her to Rome for Thanksgiving. You can simultaneously land cheap seats to great shows, genuinely help out your favorite theatres, AND show off slash tell people what to do. It's a win/win/win, really.
So there you go! Can you get away with paying less than the suggested price on a PWYC performance if the suggested price is outside your budget? YES YOU CAN. Will the cast and crew still appreciate your attendance, even if you can't afford to pay the full regular ticket price? YOU BET YOUR BROKE ASS THEY WILL, THAT'S WHAT PWYC IS FOR. Should you shout them out like crazy for making great art and then making it accessible? HECK YEAH DO THAT. If you can't show your enthusiasm with money, show it by dressing up, by bringing friends, and by promoting the shows you enjoy in your own circles. That, and show up fifteen minutes early, and don't forget to silence your cell phone.