Mitch said he bought his car off the side of the road for $100 and Toni got hers through a customer of her husband Phil's for $150.
"We've split every cost down the middle including hiring the trailer and the entry fee. I think we each spent about $430," Mitch said.
Phil has been working on the cars for a few months, stripping the cars completely and adding in the safety measures necessary when cars are intentionally smashing into each other.
Necessary modifications included the removal of all glass and any flammable material, all the extra seats, the lining and the lights.
"You just strip the whole car until it's just the outer shell," Mitch said.
"You've also got to mount five-point harness seatbelts so if you roll, you're not going to feel it.
"You've got to put in a steel bar behind the driver's seat so if you roll, it doesn't cave in."
They will also have cameras mounted inside the car to capture the carnage.
Mitch said both he and his mother had "tricks up our sleeves" to make their cars last longer in the event, but were keeping those quiet until the night.
"It's so much work for 10 minutes of fun."
Demolition Derby tonight, 6.30pm at BayPark Family Speedway.