So I was doing some thinking on this and I thought why not take more of a Zelda like approach.
What I'm getting at:
In all the Zelda games from Ocarina and up you can collect stuff and get rewards from turning them in a certain increments. Like Ocarina for example you collect gold Skulltulas and turn them in for rupees and other stuff.
So why not do the same with bugs basically for every so many you turn in you get a reward. like for 100 you get an extra 1k gralats. Then another reward at 250,500,1k ect on up to what ever amount and make a stat that keeps track of the #of bugs caught (which would be like 5 billiondy for Blueh) Or for every like 500 or 1k bugs you turn in the witch or whom ever would give you the opportunity to select from a set number of hats/items. like 3-5.
You could also get double the amount of gralats you receive for every 100th bug you turn in. So if I turn in 100 worms I would get 200 gralats. Even if you didn't turn in 100 all at once you would receive double the amount of what she was offering you at the time. So if I had turned in 75 worms earlier and was turning in 50 this time I would get 100 gralats instead of 50 for going over 100. Or perhaps you could use just this as another bonus incentive to catching and turning in bugs to help players get a little more dough.