Thelma Harper would be entitled to the full refund of only the car's purchase price--not tax, title, license, dealer prep, expenses (full tank of gas) or maintenance (car wax).
At one point, Vint tells Thelma that perhaps they need to look elsewhere when Willie shows her the expensive Cadillac, meaning Vint suspected something shady was about to happen. Yet a moment later, no one catches on to Willie's antics when he dodges Vint's questions about the Chevy Nova, especially when Willie does not permit Vint to look under the hood.