Scratch

Right guys, here's a real challange for you: (please only answer if you actually know what you're on about here).

For part of the game I'm currently working on, there's an area called mystery box. This box appears once a day, and once opened, will reveal a randomly chosen item.

Catch 1:
There is a list of possible items that the box could open to reveal, the first of which is highly probable and the last of which is the least probably (so the percentage chance of getting an item is determined by it's place in the list).

Catch 2:
Before you open the box, you have to pay. You can choose how much money you give, and this affects the probability of getting certain items! Giving the maximum amount (say, 1000 gives you the highest probability of getting an item of low probability in the list, though still offers a small chance of getting something of higher probability and less worth whilst paying the smallest amount (1) decreases the probability of getting a highly improbable item and more or less ensures getting a more probably one though there is still a possibility of gaining an improbably item).

Catch 22:
Is a pretty neat piece of work 

It was easy before catch 2 came along... does anyone have any idea how I might achive this???

Many thanks for taking on the challange!