It's an idea as American as Las Vegas (or the Philly Fringe itself). You pay your money, you pick your shows … and when it's all over, you wonder if you made the best decisions.

Welcome to the land of Pay Up, Pig Iron's interactive laboratory where money changes hands. Upon entering, you'll be given five bucks, and the game plan is explained. You'll have enough time to see six of eight short scenes, which you will choose without knowing much about them. Most cost a dollar, but inflation sets in — and with it, choices must be made. It's a lot like life: Even the richest and most driven among us can't do everything.

There's a trick, of course (well, it's a mini-economy, isn't it?): Your five dollars won't cover all six shows. You are allowed to use your own money … but if you opt out of one of the vignettes and instead stay on the sidelines, you'll find yourself entertained there, too.

Since this is the playful world of Pig Iron, there are many surprises. Don't look for Las Vegas glitz. The environment is brilliantly white and sterile (you'll put on surgical booties as you enter), and there's something a little sinister behind the Disneyland faces of the ushers and other drones who guide you from place to place.

Then there are the scenelets themselves. Most are funny, a few disturbing and some are combinations of both. All appear to be bizarre fragments of a bigger story. Connections between themes are suggested (improbably, these include dating, monkeys, greed, medical experiments and simmering power conflicts). Will we finally get the key?

Yes and no. In the end, you'll find it's an economy of ups and downs and complex backstories — but you won't know a lot more than that.

You'll be tempted to search for meaning here, but don't. Instead, let the company's tremendous cleverness and verve wash over you. I'd say that's how the Pig Iron folks themselves treat their Magic Kingdom. Much of their best work comes in several uproarious "dance breaks" between themes, where familiar tunes are retrofitted with hilarious new, financially oriented lyrics. These alone are worth (ahem) the price of admission. (The execution and performances are in every respect top-notch.)

As I exited the theater, a caring voice over the loudspeaker told me about many things I could have been doing with my entertainment money instead of seeing Pay Up. I could, for example, have seen March of the Penguins at the Ritz, or joined a cycling class at the 12th Street Gym.

Personally, I'm glad I chose Pay Up.

PAY UP Through Sept. 17, Pig Iron Theatre Company, The National Dock #2, 119 Arch St., 215-413-1318 or www.livearts-fringe.org.