Dynamic scoring system for unbalanced items?


I was wondering if there was a formula or system to dynamically change the value of certain actions or items, such that over many uses by individuals the points will balance out. For instance:
- lets say there are 6 tasks that can be completed any number of times
- they require completely different methods to complete
- you set each task to equal 10 points
- as people chose tasks to complete and complete them the points for each task will be balanced
- for tasks taken a lot the points given will decrease and for those not taken the points will increase
- over the long term the points given for each task will be different, but each task is taken at an even rate

I just want to know if there is an optimized system I can use for reference to deal with this sort of situation.