Here are some sale prices for insects I know:

Yellow/White Butterfly: 90 Bells
Honeybee: 100 Bells
Mosquito: 130 Bells
Tiger Butterfly: 160 Bells
Darner Dragonfly: 200 Bells
Firefly: 300 Bells
Emperor Butterfly: 2,500 Bells
Agrias/Birdwing Butterfly: 3,000 Bells