This Economic Policy is rarely useful, unless you really need to expand your territory quickly. The savings you'll get for purchasing a single tile are hardly worth it the opportunities you'll lose when not using other Economic Policies.
Adopting this Policy for a short time may be justified when you Found a new City close to a competitor, and you need to quickly expand to include tiles that otherwise may be taken by him. It is also handy when you have low Culture production and your cities keep expanding to the wrong tiles. Make sure you have the necessary Gold, though, or that you will get it shortly (in the time it takes to develop your next Civic); otherwise you are again wasting opportunities.
Civilopedia Entry Edit
Once the idea of ownership of the land was ingrained in humanity, methods for determining boundaries were necessary. In ancient Egypt, a rope with knots could be used with simple geometry to re-establish boundaries after the annual floods along the Nile. Land surveyors were also at work around those old monuments tourists are now enamored of; Stonehenge was laid out by surveyors using pegs and ropes, as was the Great Pyramid of Giza. The Chinese Liu Hui described methods of surveying in works published c. 263 AD. But it was the Romans who first acknowledged land surveying as a profession; there, surveyors were known as Gromatici, so named after the Groma instrument dating back to ancient Mesopotamia.