For your event, points should be based off of price rates.

Not sure if this is a credible source, but base it off something like this.
http://www.graalians.com/forums/show...catching+guide

The rarer the bug the more points they should be, the more they spawn the less points they should be. The difficulty to catch, the more/less points, since some butterflies even seem faster than others of the same type.