Ralestone Luck by Andre Norton

Once upon a time two brave princes and a beautiful princess set out to make their fortunes- began the dark-haired, dark-eyed boy by the roadster. "Royalty is out of fashion," corrected Ricky Ralestone somewhat indifferently. "Can't you do better than that?" She gave her small, pert hat an exasperated tweak which brought the unoffending bowl-shaped bit of white felt into its proper position over her right eyebrow. "How long does it take Rupert to ask a single simple question?" Her brother Val watched the gas gage on the instrument board of the roadster fluctuate wildly as the attendant of the station shook the hose to speed the flow of the last few drops. Five gallons-a dollar ten. Did he have that much? He began to assemble various small hoards of change from different pockets.