be a license to print money
To yield very high profits but require little or no effort to do so, as of an activity, business model, company, etc. All these hefty parking fees and fines are just a license to print money for the city. These trashy gossip magazines are of such low quality, yet they always fly off the shelves. The whole racket is a license to print money. A wildly popular film franchise is basically a license to print money.
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