Once you get into the area where the Thermal Bunyip is located, run towards the left. 
There will be a 500 Opal bag. Go back out the way that you came in. You can get the 
door leading to the exit to open for you. Simply walk out the door, turn around, and 
go back into the room with the 500 Opal bag and pick it up again. Repeat this process 
as many times as desired.

Boomerang costs
BOOMERANG-FREE
FLAMERANG-2,000
FROSTYRANG-2,000
LASSORANG-4,000
ZAPPYRANG-4,000
INFRARANG-4,000
SMASHARANG-5,000
MEGARANG-6,000