If you go to www.wonga.com, you can fiddle with the sliders and see how much a payday loan costs in the UK. For instance, if you borrow £100 for 10 days, you're paying back £115.91, which is just over 1% per day. I can definitely imagine someone accepting that to pay for an urgent cost.

I expect that if you don't repay when you agreed, though, you're going to end up paying a lot more.