Your cousin says that you can shoot in his apartment for free. But it's a studio. With a cat. And loud neighbors.
The favor game starts like this:
PRODUCER: Why is the location rentals line item so high?
ME: You have three days of shooting at a diner.
PRODUCER: I have a [insert relationship] with a [insert location] who can get us this for [insert ridiculously low price]. So just take that diner out, okay?
ME: [Reluctantly complies]
Free locations, cars, extras, gear, personnel, etc. And sometimes it works out. But each favor has hidden costs that the producer seldom considers. So how do you budget favors?
LOCATIONS: SIZE AND DISTANCE MATTER
If the producer's aunt's log cabin is perfect for the horror movie, that's great. But is it big enough for everything else - holding, equipment, staging, parking, eating? If not, you still have to find a place (preferably nearby) for some or all of these things. Is it furnished? Is the electricity working?
Is it more than an hour's drive away from the central pick-up point? Then you may go into overtime each day (typically, travel time over an hour is "on the clock" for the crew and cast). You'll also have turnaround issues. And your gas and tolls expenses will go up. Or, you'll have to feed and house people somewhere less than an hour away.
FREE / LOW COST CREW
This depends on two things: the experience level of the members, and their relationship to the person asking the favor. A DP who brings her gaffer along on all the really nice-paying commercials can get a rate break from him for your low-budget feature. People have given me breaks because I've referred them for jobs in the past. But first-time directors and producers often don't have those kinds of favors, so they rely on their somewhat untested friendships with greener crew.
A green crew will sometimes take longer to get up to speed, so you should schedule more days (which makes other costs go up). You may still come out ahead, however, so you shouldn't worry about this too much.
On the other hand, people with absolutely no film experience can present problems. They don't realize how difficult the work is. On a typical "favor crew" shoot a few years ago, I started out the first week with 25 people in the crew and ended it with 15. I'm not sure I saved any money by having 10 more mouths to feed those first few days.
This depends on where you're shooting and the condition of the gear. When shooting in a city with a dozen rental houses, I'm not worried if the DP's friend's camera stops working, because I can get a replacement and keep shooting. But if you're in the boonies, you'd better hope that your friend maintained that camera. One compromise is to budget for a checkout/test day, where the AC/gaffer/sound mixer/etc. tests the gear before you start shooting.
FREE PROPS, WARDROBE, ETC.
Just budget some kind of replacement cost. Somehow it's always the borrowed stuff that comes back scuffed, torn, or crushed.
Favors are great, and they can make the difference between a film getting made and a script sitting on a hard drive. Just try to budget the hidden costs, and make sure you're getting real value.