One place where my wife managed was $75 for $500 loan. So the repayment was $575. But I can't remember if repayment was in 30 days or just the next paycheck.
They're fine if you can commit to repaying in full right off the bat, and don't loan hop. What gets a lot of morons in to trouble is they'll hit up 3 or 4 or 5 different payday loan companies in one day. It immediately catches them in an impossible cycle because, typically, someone in that demographic doesn't earn enough to pay each of them back plus fees. While the loan companies do credit and background checks and all that, they have no way of knowing if you've hopped around to 5 other places that day.
Ignore the APR (which is often like 500+%), because they're not meant to be loans longer than a paycheck or two. Think of it as a fee. It costs you $X per month to borrow $Y from the PayDay lender. You can continue paying $X per month until you've paid back $Y (aka "rolling over")....which is the same as just throwing away $X every month. Multiply that by 5 places and you understand how fools get themselves trapped.
The reason it's setup that way is because of the inherently high risk pool of the people taking out these loans and not paying them back, bankrupting, skipping town, etc... OTOH, there do exist people with legitimate need for quick cash-in-hand an no one else to turn to. Like, say, a broken down car which gets you to/from your job.