Provisional patent application filing is a very simple procedure: this is not a full formal patent application. If the client supplies us with a disclosure document which contains enough information, we can file a minimal provisional application for (typically) less than $1000 all told, including the government fee. The requirements for the disclosure document are:
1) It must "enable" other technologists to "make and use the invention."
2) It must contain at least one drawing (and preferably more). (In theory this is sometimes not required, but we recommend that a disclosure should illustrate the invention as well as possible.)
The disclosure document does not have to be written in any particular format, and does not have to contain claims. The government fee does not have to be paid at the time of filing.
Note that filing a "minimal" provisional application is a limited engagement for us. A full patent application write-up is undoubtedly better, but our fees for preparing a full patent application would typically run in the neighborhood of $10K - maybe $5K for a very very simple mechanical invention, sometimes $20K or more for a complicated software or electrical or business method invention. However, our view is that it is far better to file a minimal provisional application than none.
Filing a provisional application is not the end of the process. A further filing must be made within one year of the first provisional filing, since the provisional application itself will expire. This deadline applies to US non provisional filings, international (PCT) filings, and Taiwanese national filings. For example: if a first provisional is filed on March 8, 2013, and an expanded provisional is filed in June 2013, the absolute deadline for further filings will be March 8, 2014.