Monty Python and the Holy Grail (1975)

Monty Python and the Holy Grail (1975) was made for just £229,000, using coconut shells instead of horses to save costs, yet earned over $229 million in lifetime revenues — a 1,000x return on investment that outperforms #7 on this list. It spawned the Tony Award-winning musical Spamalot (2005) and was named the 5th greatest British film by the British Film Institute, cementing its iconic status.