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 upgrades
Zapyrang: Thunderang
Boomerang: Multirang
Flamerang: Lavarang
Frostyrang: Freezerang
Infarang: X-rang
Megarang: Omegarang
Smasherang: Kaboomerang
Lasharang: Warparang

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
FREEZERANG: 8,000
LAVARANG: 8,000
MULTIRANG: 8,000
KABOOMERANG: 12,000
OMEGARANG: 14,000
DEADLYRANG: 30,000
DOOMERANG: 75,000
X-RANG: ?