HI
I agree with one idea put forth... add a point bonus based on size of collection.
Perhaps as the collection increases in size points would increase. A collection that is 5% complete would include a 5% bonus in points. A 10% complete collection a 10% bonus. This would reward each step of the way. Then perhaps as the collection is nearly complete a bonus would begiven at 96%, 97%, 98% etc. The points would be percentages of the points you have earned. A higher graded collection would receive a percent of the points they have, not just a set amount of points. Everyone likes to see growth and reward for efforts.
Regards
Steve