I believe the guys that made SAW used this technique, they shot an entire scene and got financing in place. The best thing to do with a pre-sales trailer is to get pre-sales from buyers and skip investors all together. If you can get 2-3 pre-sales this may be enough to finance the entire movie. You take the pre-sales contracts to a bank that does film deals and have them advance you the amount of the contracts in the form of a production loan. You use the loan to make the movie and pay your self a salary. When the movie is done, you ship it, and the the distributors pay off the bank loan.
I can tell you that dozens of movies are made this way, however most of these deals are made by experienced producers with a proven track record with the bank and the buyers. The bank will want to make sure that the buyers are 100% rock solid companies. For instance a contract from an obscure distributor in the UK won't be worth any thing to a bank, but a contract from the BBC will be quite solid in the eyes of the bank.
Also if you live in a state that has a tax credit program like Michigan or New Mexico you can also get the bank to advance you your tax credit refund based on your budget, for instance if you are getting a tax credit refund of 200K, the bank may advance you 175K against this amount. Again you need to find the right bank, most banks run away in horror when you say the word "film."
If you can get 2-3 solid pre-sales contracts and a solid tax credit estimate, you can bank roll the entire movie and be working with a much bigger budget than you had anticipated. Many filmmakers are better off raising up their budget and attaching a known lead actor in order to secure the pre-sales. This will also trigger a higher tax credit.
All of this can be combined into the films production account at the bank and you are ready to roll.
This may sound tough, and it is, but the fact is many movies are made under this scenario. The other problem is that most filmmakers are more interested in the creation of the movie and less interested in the hard core business skills needed to pull off a deal under this structure. This is where it can be a good idea to bring in a producer who has experience in this area so you can concentrate on the creative aspects of the film.
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