http://www.paessler.com/blog/2015/04/07/all-about-prtg/paessler-offers-prtg-100-for-free
That was a 330 euro license
Obviously there is open source out there but PRTG is an absolute doddle to get going. The Cloud Ping sensor is very handy. You run your own single server install of PRTG but can make use of their cloud setup, so can get ping results from 5 continents.
That's why we now give away the 100 sensor license to everybody, for example, small business owners, startups, and private users. And if your business grows you don't have to change your monitoring solution—just upgrade to one of our commercial licenses with more sensors.
That was a 330 euro license

Obviously there is open source out there but PRTG is an absolute doddle to get going. The Cloud Ping sensor is very handy. You run your own single server install of PRTG but can make use of their cloud setup, so can get ping results from 5 continents.