Who makes more money, .NET developers or open-source platform developers?

It depends on the developer skills.

Here in Jordan/ Middle East - open source developers make more money, I think this related to business natural.

Open source is a wide range of technologies and are not used in the same way in all countries.
PHP developer are generay less payed than .NET developers, wheras java developers are more payed in coutries where open source plateformes are widely used (like France for instance)

it depends.
open source are mostly used by start ups and small-mid level companies these companies are in magnitude.
those who care outsourcing on the sites like odesk etc are also making good chucks of money.
.net is more of an enterprise level solutions and mostly implemented in big companies, so people working at these big MNCs are making a good salary.