The main cost involved in shooting on film is the film stock, processing, and transfer (assuming you're going to edit digitally) costs.
Lets do some basic math here.. about the cheapest you'll find 16mm film stock is
$16 per 100' re-canned roll.
Not counting leader and tail lengths that will be unusable, that 100' will get you roughly 2 3/4 minutes of footage at 24fps. For processing and transfer, figure somewhere between $.50 and $1.00 per foot, so that's at least another $50, but lets be ultra generous and say you find a smoking deal and get it for half price.. so now between film stock, processing, and transfer to digital you're sitting at about $40 per 2 1/2 minutes of footage.
Lets assume you're shooting a 5 minute short film. For the sake of this scenario lets say you're able to achieve a 6:1 shooting ratio. So, you'll shoot 30 minutes of footage to cut down to that 5 minute short (this is a very conservative estimate). That puts you at just under $500 for your film stock, processing, and transfer. Of course that's not including shipping charges, extra fees the lab charges for processing and transfer, etc..
But, $500 for 30 minutes of footage. On the other hand, you could get twice that amount of footage for about $10 on a very high quality tape (or $5 if you buy the nice bargain maxell tapes like I do) and you'd have the rest of that $500 to put into food for your cast/crew, renting extra lights and grip equipment, fully outfitting your locations, etc..
Now I realize not all of that extra stuff applies to your short that takes place in the park, but it should be pretty clear that film is significantly more expensive than video.