Here's how it works:

Associated with each prize is a probability in which it can appear.

if the probability value is 1000000 it will appear every time
if the probability is 1 in a million.

The lower value prizes are assigned a probability of 750000 or so
The high value prizes get a probability of ABOUT 250000 (I IN 4 CHANCE)
The really high value stuff gets a probability of 1000 or less.
If your luck sucks and you get none of them there is a default prize handed out (i.e a ticket).

So is it rigged? Yes, insofar as the probability that you will get anything of decent value for your RPs. But the actual draw itself is random.