Most of the other filmmakers are asking you to delineate what exactly defines cost. If our hypothetical filmmaker owns a laptop on which s/he wrote the script it is no longer without an investment on the part of our hypothetical filmmaker. Once the script is printed our notional filmmaker has "invested" in paper. So at the very least the budget includes a laptop and a few reams of paper.
I think that Smitty (Smithproductions) is asking how to make a film without any personal investment from prepro onward. The hypothetical filmmaker he poses wants everyone to donate time, equipment & facilities, transportation, props, costumes, make-up, locations, food & drink, etc., etc., etc., etc. so that the hypothetical filmmaker can preproduce, shoot, post and market their script so there is no personal cost to him/her beyond writing the script.
Sure, it's possible, but extremely improbable.
I work once or twice each year with a producer on a pro bono basis doing shorts (plus a few paying jobs). The freebees always involve very talented people. As an example, on one for free project our DP did corporate and wedding work, the editor did corporate and advertising work, the PSM was a very ambitious up-and-comer with some decent work under his belt. The lighting was by a pro photographer looking to branch out into film/video, the wardrobe/H/MU team were recruited from local beauty salons. The actors are always top-knotch. (As an aside, besides the usual places, our producer looks at big-budget films and TV shows shot in the NY Tri-State area and contacts speaking-part extras who might fit whatever project is in the works.)
So yes, all these people work for free and supply a majority of the equipment needed for the shoot and for post. However, our producer treats everyone like the professionals they are or aspire to be. Transportation costs are covered, and an extremely lavish craft table and meals are provided. This includes preproduction, where all the dept. keys, talent, director, screenwriter and producer are present in a very relaxed beer/wine & knoshes roundtable discussion atmosphere. Most important of all, everyone is vigorously encouraged to contribute during prepro. On one project our location/sets/props gal made suggestions to a script that took us from six (6) to only two (2) locations. On another project I made a suggestion that caused a partial rewrite of the script. This type of involvement gives everyone the feeling that they are involved in a creative process, not just a bunch of hired hands.
It also means that we all get to work with (and add to our networks) some very talented people, and, because of the type of people the producer recruits, quite compatible temperamentally and professionally.
So on a short the producer will spend a few grand on food, liquids and transportation plus whatever minimal equipment (rented) and props may be required, as well as replenishing the supplies of the H/MU team. A very minimal investment considering the high quality of the final products.
The whole point is that something needs to be offered to the participants to make the for free project a worthwhile investment of their time and talents. If our hypothetical filmmaker will not spend any money at all, even to treat the cast and crews like professionals, I give notional feature a 99.9999% chance of failure.