Mate, good luck, I have been thinking about the same thing for a while now but the problem is that prices for NRL cards are subjective, I mean they are only worth what someone is willing to pay for them but I agree with the school of thought that as your collection grows that has to be an increase in value as it becomes more complete.
What I am going to do is this, start with my 1988 base set and research a replacement value, do that for all the base sets / albums then factor in the inserts / auto's etc. You have to be realistic with the value to replace by that I mean you might pay 200 dollars for a 1996 all fired up Brad Fittler auto (3 card set) but another could go for 80 dollars, in that case I will pick a figure around the middle of say 120 - 130 dollars and will give that to all the '95 and '96 auto's.
Hope that helps some.
Dave