But it doesn't get consumed in the process -- the same gallium can be reused infinitely. So, it's a one-time cost in the manufacturing of the fuel system, rather than a recurring cost. Also, it doesn't have to be high-grade gallium, a fair bit of it can come from the byproducts of the semiconductor industry.
There is valuable metal in the catalytic converter of the exhaust system. When they first started being used it was estimated they would last 50,000 miles and have to be replaced, not cheap, even though the catalyst was recoverable. Likely the gallium would have to be recycled, so the entire unit would be replaced when the aluminum is oxidized. If they start putting gallium in cars, the quantity would be large. How much would come from the leftovers of the semiconductor industry?