My first piece of evidence for the proposition that employers pay for non-competes is that some of them prefer to hire employees with non-competes. Companies pay for everything they require of employees, as all unnecessary conditions reduce their options (for employees), and the only reason they ever offer money is to increase their options (from 0-1 or 100-200). In addition, it is very common for employers to value non-competes as assets (for accounting purposes). There are many other points in favor of the idea that companies do pay for non-competes, but we can start here.
I attribute the downvotes to the unpopularity of non-complete clauses on HN. In addition to that, most of the libertarian and right-leaning users seem to be in North America, so I can count on less sympathy from ~4-12 GMT. It is also possible that my writing has gotten substantially worse in the last few hours, but it seems unlikely that I am being more rude or unfair than I normally am.
The non compete is nearly entirely valuable in that it it depresses wages. In reality its entirely likely that between firm A and firm B neither has a whole lot of special sauce non competes mostly serve to keep each other from competing for employees thus driving up wages.
Why would you suggest that the employee pay a cost for accepting a fair workers agreement (no non-compete clause)? At least suggest that salaries be raised for people on non-competes.
The value the employee is generating through this IP is always going to be considerably (orders of magnitude) larger than whatever their salary is - the company can afford to pay them more.
In fact its trivial to show that non competes result in depressed wages. It in fact shows you the exact opposite of what you intend to show. Non competes are valued as assets as it enables employers to pay employees LESS than in a free market.