Probabilities and random numbers

I would like some assitance in a little probabilities script I’m writing. Currently I have an array that selects one out of four images for display. However, I’m augmenting this script as a scratch and win type of application. Thus the user will be able to scratch off a mask and unveil thier prize. However, I wanted to have varying degrees of prize availability per use.

Prize 1: 70%
Prize 2: 15%
Prize 3: 10%
Prize 4: 5%
Chances of it coming up

Now this code needs to be re-adjusted for future use so that it can quickly re calculate probabilities say if a 5th prize were to be added.

The option I considered was to have a random 1-4 number generator and then if it was number 4 then re do another random generator 1-100 and if it was 1-5 then they get prize 4 else automatically prize 1.

But this is not effective method for future use. It needs to be a script that can be enhanced and added onto and have prizes removed/percentages altered/prizes added easily rather then a set of next code to do another random num generation.

Any help or scripts would be appreciated. Also if can be done through xml that would be great too and pass the prize value to the flash file as currently all the images of the prizes are loaded through xml.