Anyway, here is
There is no need to pay for gym membership if you wanted to run. No need to rent out basketball courts, soccer fields, tennis courts, badminton courts if you wanted to run!
You don't have to pay for a yoga instructor, don't have to pay fee for use of swimming pool and you don't need any fancy club membership to run! No need to pay for boxing/ tae kwon do or other martial arts instructors to run!
All you need is a road to run on, a pair of running shoes, shorts or pants and shirt, and you are all set!
Name another sport/ exercise that is as cost effective as this! And if you named biking.... well, you still need a good bike in order to do that activity and that can put a big dent to your wallet as well.
The nearest thing I could think of is rollerblading/ rollerskating (and isn't that like running but with wheels/blade)!