Terry,
Some governments already do sell citizenship. But that's just the beginning of the possibilities that border controls enable! Charging people to cross national borders, and selling various rights to those who are not granted them automatically, is a huge potential money-maker for governments. People want to enter and leave countries for various reasons (vacations, work, migration, temporary residence, medical visits, etc.), and many of them are no doubt willing to pay a substantial fee for these "privileges". Large numbers of governments already force visitors to buy special documents (visas) in order to enter the countries they control. Sometimes there are different fees for different privileges -- so much to come here as a student, so much to come here to work, etc. The Cuban regime simply forces people entering the country to buy a certain amount of their grossly over-valued currency.
But for the government of a large, wealthy country, charging entry is certainly not the only way that border controls and citizenship rules can be profitable. The U.S. government appears to be slowly realizing this. They have now effectively made it mandatory to have a U.S. passport in order to leave the country. You can still physically get out without one in some cases, by going into Canada or Mexico, but you can't readily get back in without one. My understanding is they have the cooperation of other governments in this -- i.e. you may be able to get on an international flight without one, but you wouldn't be allowed to disembark at the other end.
Barring major pro-freedom political change, I expect that simply going into Canada or Mexico without a passport will become illegal sometime soon. How quickly this happens may depend on how much the nationalists' desire for border controls to be ramped up is fulfilled -- the more money and personnel are put into controlling borders, the more readily such a rule could be conveniently enforced.
Before 1941, U.S. citizens generally did not have to have a passport in order to travel outside the country (see http://www.archives.gov/genealogy/passport/). Since then, the cost of obtaining and renewing a passport has been steadily increasing. A new adult passport now costs $165, and if you want one more quickly, there are higher fees you can pay to help make that happen (see http://travel.state.gov/passport/fees/fees_837.html).
Raising the cost of passports has an advantage for governments beyond the direct revenue stream from people buying them -- it helps keep tourism dollars, and therefore tax revenue, inside the country. Just think of it as a massive "buy local" program. Heck, they could even sell it as a way to help the environment, by reducing consumption of jet fuel and reducing the impact on natural habitats overseas that may not be as well protected as those within the United States.
When it gets expensive enough, and therefore desirable enough, to get permission to leave the United States -- perhaps a passport good for several years will cost $10,000 instead of $165 -- the U.S. government will theoretically be able to regulate the effects of immigration relatively simply, by letting a certain number of poor people exit for free or at reduced rates, in order to balance the number of people arriving. If promoted sensibly, this might make immigration less of a "third rail" topic in the political arena.
Perhaps future Libertarians in popular tourism destination countries, like France and Spain, will see the U.S. government charging people massive amounts of money to leave the United States as a sensible, market-based solution to the problem of "their" countries being overrun by budget tourists, ensuring that most of those entering "their" countries bring lots of money to spend without having to get "their own" hands dirty by turning away the poor. Heck, as Derek's recent California border control encounter illustrates, such rules could also be profitably implemented between U.S. states. "You're free to enter California, but you can't bring your vehicle or any baggage with you unless you obtain a release from an inspector. There is a $100 per person inspection fee."
Or maybe (dare I hope?) people around the world will eventually start to realize what a racket and a threat to freedom it is for governments to maintain border controls in order charge people fees (taxes) in order to enter or leave the areas they control or to enjoy equal rights in those areas, no matter which groups of people are initially targeted or burdened.
But who am I kidding? Those who are wealthy or privileged enough that the controls don't negatively impact them very much will readily find classist and xenophobic rationalizations to support them, and generally speaking those are the people who have the political power to influence what rules governments set. The rest of us will have the choice of staying where we are, or taking the risky option of breaking the rules.
Oh wait, I forgot we on this list are still relatively wealthy and privileged and aren't part of "the rest of us" yet. The really tough, expensive choices are currently for those "south of the border", not "us" who fortunately live "north of the border". Never mind, carry on.
Love & Liberty,
((( starchild )))
P.S. - Another possibility, if nationalists were to succeed in repealing the 14th Amendment -- or if the federal government simply starts selectively ignoring it, as they do so much of the Constitution -- is that citizenship, instead of being a birthright and guaranteed for life, could be made renewable for a fee every few years. Not only could this be a major source of revenue, but it would help catch criminals, make it easier to prevent undesirable people from voting, etc. You don't want child molesters and terrorists voting, do you? Then let's start working to erode the concept of birthright citizenship!