For example, fre market economists are usually pro economic immigration: Let resources (labour) go to their highest value use. If a graphic designer can make more in an Oslo design office than they can in Bucharest, there is a potential economic surplus. Opponents might make some objections: there will be increased unemployment among local graphic designers, the immigrants will mean increased costs in local services.

The economist-proponent looks at the surplus. Going from a €7k salary to €50k. Adjusted for costs, he calculates a €25k surplus. All the opponents objections can be overcome with €5k at most: new job training centres, schools and hospitals. He throws in another €5k to make the argument stick. He can have his immigration and unemployment will go down, hospitals & schools will be better funded, design firms will have better/cheaper designers and the immigrant will have a better income - even after they charge him €10k a year.

Makes sense on paper. It's a good way of simplifying an argument by creating an on paper ceteris parabus. When it actually makes it into policy ir law, it can be arbitrary and unfair at the best of time (eg immigrants pay higher taxes). It's also a potential faucet for funding that the hungry will flock to.

The weed smoker has a lot to gain. Lower prices. Better quality. Reduced stigma. Easier access. Not getting put locked a cage. Well worth the $500 per year.